Heap use-after-free in xmlSAX2AttributeNs

For https://bugzilla.gnome.org/show_bug.cgi?id=759020

* parser.c:
(xmlParseStartTag2): Attribute strings are only valid if the
base does not change, so add another check where the base may
change.  Make sure to set 'attvalue' to NULL after freeing it.
* result/errors/759020.xml: Added.
* result/errors/759020.xml.err: Added.
* result/errors/759020.xml.str: Added.
* test/errors/759020.xml: Added test case.
diff --git a/parser.c b/parser.c
index 15c606f..7aba6a9 100644
--- a/parser.c
+++ b/parser.c
@@ -9488,7 +9488,10 @@
 		else
 		    if (nsPush(ctxt, NULL, URL) > 0) nbNs++;
 skip_default_ns:
-		if (alloc != 0) xmlFree(attvalue);
+		if ((attvalue != NULL) && (alloc != 0)) {
+		    xmlFree(attvalue);
+		    attvalue = NULL;
+		}
 		if ((RAW == '>') || (((RAW == '/') && (NXT(1) == '>'))))
 		    break;
 		if (!IS_BLANK_CH(RAW)) {
@@ -9497,6 +9500,8 @@
 		    break;
 		}
 		SKIP_BLANKS;
+		if ((ctxt->input->base != base) || (inputNr != ctxt->inputNr))
+		    goto base_changed;
 		continue;
 	    }
             if (aprefix == ctxt->str_xmlns) {
@@ -9568,7 +9573,10 @@
 		else
 		    if (nsPush(ctxt, attname, URL) > 0) nbNs++;
 skip_ns:
-		if (alloc != 0) xmlFree(attvalue);
+		if ((attvalue != NULL) && (alloc != 0)) {
+		    xmlFree(attvalue);
+		    attvalue = NULL;
+		}
 		if ((RAW == '>') || (((RAW == '/') && (NXT(1) == '>'))))
 		    break;
 		if (!IS_BLANK_CH(RAW)) {
diff --git a/result/errors/759020.xml b/result/errors/759020.xml
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/result/errors/759020.xml
diff --git a/result/errors/759020.xml.err b/result/errors/759020.xml.err
new file mode 100644
index 0000000..a0d3051
--- /dev/null
+++ b/result/errors/759020.xml.err
@@ -0,0 +1,6 @@
+./test/errors/759020.xml:3: namespace warning : xmlns: URI 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 is not absolute
+0000000000000000000000000000000000000000000000000000000000000000000000000000000'
+                                                                               ^
+./test/errors/759020.xml:46: parser error : Couldn't find end of Start Tag s00 line 2
+                                                                   
+                                                                   ^
diff --git a/result/errors/759020.xml.str b/result/errors/759020.xml.str
new file mode 100644
index 0000000..998d6d2
--- /dev/null
+++ b/result/errors/759020.xml.str
@@ -0,0 +1,7 @@
+./test/errors/759020.xml:3: namespace warning : xmlns: URI 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 is not absolute
+0000000000000000000000000000000000000000000000000000000000000000000000000000000'
+                                                                               ^
+./test/errors/759020.xml:46: parser error : Couldn't find end of Start Tag s00
+                                                                   
+                                                                   ^
+./test/errors/759020.xml : failed to parse
diff --git a/test/errors/759020.xml b/test/errors/759020.xml
new file mode 100644
index 0000000..db23275
--- /dev/null
+++ b/test/errors/759020.xml
@@ -0,0 +1,46 @@
+<?l 00000000000000000000000000000?>
+<s00 w0000="000" h00000="000"
+ xmlns = '00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000'       
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