Fix large parse of file from memory

https://bugzilla.redhat.com/show_bug.cgi?id=862969
The new code trying to detect excessive input lookup would
just get wrong sometimes in the case of very large file parsed
directly from memory.

libxml.h

@@ -91,6 +91,8 @@ void __xmlGlobalInitMutexDestroy(void);
 int __xmlRandom(void);
 #endif
 
+int xmlNop(void);
+
 #ifdef IN_LIBXML
 #ifdef __GNUC__
 #ifdef PIC