Cleanup conditional section error handling

For https://bugzilla.gnome.org/show_bug.cgi?id=744980

The error handling of Conditional Section also need to be
straightened as the structure of the document can't be
guessed on a failure there and it's better to stop parsing
as further errors are likely to be irrelevant.
diff --git a/parser.c b/parser.c
index bbe97eb..fe603ac 100644
--- a/parser.c
+++ b/parser.c
@@ -6770,6 +6770,8 @@
 	SKIP_BLANKS;
 	if (RAW != '[') {
 	    xmlFatalErr(ctxt, XML_ERR_CONDSEC_INVALID, NULL);
+	    xmlStopParser(ctxt);
+	    return;
 	} else {
 	    if (ctxt->input->id != id) {
 		xmlValidityError(ctxt, XML_ERR_ENTITY_BOUNDARY,
@@ -6830,6 +6832,8 @@
 	SKIP_BLANKS;
 	if (RAW != '[') {
 	    xmlFatalErr(ctxt, XML_ERR_CONDSEC_INVALID, NULL);
+	    xmlStopParser(ctxt);
+	    return;
 	} else {
 	    if (ctxt->input->id != id) {
 		xmlValidityError(ctxt, XML_ERR_ENTITY_BOUNDARY,
@@ -6885,6 +6889,8 @@
 
     } else {
 	xmlFatalErr(ctxt, XML_ERR_CONDSEC_INVALID_KEYWORD, NULL);
+	xmlStopParser(ctxt);
+	return;
     }
 
     if (RAW == 0)