integrated the Out Of Memory test from Havoc Pennington #109368 a lot of

* Makefile.am testOOM.c testOOMlib.[ch] : integrated the Out Of
  Memory test from Havoc Pennington #109368
* SAX.c parser.c parserInternals.c tree.c uri.c valid.c
  xmlmemory.c xmlreader.c xmlregexp.c include/libxml/tree.h
  include/libxml/parser.h: a lot of memory allocation cleanups
  based on the results of the OOM testing
* check-relaxng-test-suite2.py: seems I forgot to commit the
  script.
Daniel

xmlmemory.c

@@ -305,6 +305,8 @@ xmlReallocLoc(void *ptr,size_t size, const char * file, int line)
     unsigned long number;
 
     if (!xmlMemInitialized) xmlInitMemory();
+    if (ptr == NULL)
+        return(NULL);
     TEST_POINT
 
     p = CLIENT_2_HDR(ptr);