[libxml2] Check for custom free function in global destructor

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 956534e02ef280795a187c16f6ac04e107f23c5d
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Tue Aug 4 19:27:13 2020 +0200

    Check for custom free function in global destructor
    
    Calling a custom deallocation function in the global destructor could
    cause all kinds of unexpected problems. See for example
    
        https://github.com/sparklemotion/nokogiri/issues/2059
    
    Only clean up if memory is managed with malloc/free.

 parser.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)
---
diff --git a/parser.c b/parser.c
index e1d139d5d..c66dbae47 100644
--- a/parser.c
+++ b/parser.c
@@ -14696,7 +14696,12 @@ xmlCleanupParser(void) {
 static void
 ATTRIBUTE_DESTRUCTOR
 xmlDestructor(void) {
-    xmlCleanupParser();
+    /*
+     * Calling custom deallocation functions in a destructor can cause
+     * problems, for example with Nokogiri.
+     */
+    if (xmlFree == free)
+        xmlCleanupParser();
 }
 #endif