fix: avoid segfault at exit when using custom memory functions

This extends the fix introduced by 956534e to Windows processes
dynamically loading libxml2.

Closes #256.

parser.c

@@ -14672,6 +14672,7 @@ xmlInitParser(void) {
 	return;
 
 #if defined(_WIN32) && (!defined(LIBXML_STATIC) || defined(LIBXML_STATIC_FOR_DLL))
+    if (xmlFree == free)
         atexit(xmlCleanupParser);
 #endif