[libxml2] Fix parser termination from "Double hyphen within comment" error

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 3c0d62b4193c5c1fe15a143a138f76ffa1278779
Author: David Warring <david warring gmail com>
Date:   Mon May 13 07:15:44 2019 +1200

    Fix parser termination from "Double hyphen within comment" error
    
    The patch fixes the parser not halting immediately when the error
    handler attempts to stop the parser.
    
    Rather it was running on and continuing to reference the freed buffer
    in the while loop termination test.
    
    This is only a problem if xmlStopParser is called from an error
    handler. Probably caused by commit 123234f2. Fixes #58.

 parser.c | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/parser.c b/parser.c
index 5879d3bc..2cab92c6 100644
--- a/parser.c
+++ b/parser.c
@@ -4955,6 +4955,10 @@ get_more:
                } else
                    xmlFatalErrMsgStr(ctxt, XML_ERR_HYPHEN_IN_COMMENT,
                                      "Double hyphen within comment\n", NULL);
+                if (ctxt->instate == XML_PARSER_EOF) {
+                    xmlFree(buf);
+                    return;
+                }
                in++;
                ctxt->input->col++;
            }