autogenerate a minimal NULL value sequence for unknown pointer types This

* gentest.py testapi.c: autogenerate a minimal NULL value sequence
  for unknown pointer types
* HTMLparser.c SAX2.c chvalid.c encoding.c entities.c parser.c
  parserInternals.c relaxng.c valid.c xmlIO.c xmlreader.c
  xmlsave.c xmlschemas.c xmlschemastypes.c xmlstring.c xpath.c
  xpointer.c: This uncovered an impressive amount of entry points
  not checking for NULL pointers when they ought to, closing all
  the open gaps.
Daniel

parser.c

@@ -10384,6 +10384,8 @@ xmlParseCtxtExternalEntity(xmlParserCtxtPtr ctx, const xmlChar *URL,
     xmlChar start[4];
     xmlCharEncoding enc;
 
+    if (ctx == NULL) return(-1);
+
     if (ctx->depth > 40) {
 	return(XML_ERR_ENTITY_LOOP);
     }