Fix SEGV in xmlSAXParseFileWithData

Fixes #181.
diff --git a/parser.c b/parser.c
index be14c32..f779eb6 100644
--- a/parser.c
+++ b/parser.c
@@ -14077,7 +14077,7 @@
 
     if ((ctxt->wellFormed) || recovery) {
         ret = ctxt->myDoc;
-	if (ret != NULL) {
+	if ((ret != NULL) && (ctxt->input->buf != NULL)) {
 	    if (ctxt->input->buf->compressed > 0)
 		ret->compression = 9;
 	    else