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

uri.c

@@ -1992,6 +1992,9 @@ xmlCanonicPath(const xmlChar *path)
     }
 
     uri = xmlCreateURI();
+    if (uri == NULL) {
+        return(NULL);
+    }
 
 #if defined(_WIN32) && !defined(__CYGWIN__)    
     len = xmlStrlen(path);