a lot of small cleanups based on Linus' sparse check output. Daniel

* HTMLparser.c SAX2.c encoding.c globals.c parser.c relaxng.c
  runsuite.c runtest.c schematron.c testHTML.c testReader.c
  testRegexp.c testSAX.c testThreads.c valid.c xinclude.c xmlIO.c
  xmllint.c xmlmodule.c xmlschemas.c xpath.c xpointer.c: a lot of
  small cleanups based on Linus' sparse check output.
Daniel
diff --git a/valid.c b/valid.c
index f4e05f6..32132fa 100644
--- a/valid.c
+++ b/valid.c
@@ -453,14 +453,14 @@
     xmlNodePtr ret;
 
     if (ctxt->nodeNr <= 0)
-        return (0);
+        return (NULL);
     ctxt->nodeNr--;
     if (ctxt->nodeNr > 0)
         ctxt->node = ctxt->nodeTab[ctxt->nodeNr - 1];
     else
         ctxt->node = NULL;
     ret = ctxt->nodeTab[ctxt->nodeNr];
-    ctxt->nodeTab[ctxt->nodeNr] = 0;
+    ctxt->nodeTab[ctxt->nodeNr] = NULL;
     return (ret);
 }
 
@@ -3866,7 +3866,7 @@
 	
 	fullname = xmlBuildQName(elem->name, elem->ns->prefix, fn, 50);
 	if (fullname == NULL)
-	    return(0);
+	    return(NULL);
 	attrDecl = xmlGetDtdAttrDesc(doc->intSubset, fullname, name);
 	if ((attrDecl == NULL) && (doc->extSubset != NULL)) {
 	    attrDecl = xmlGetDtdAttrDesc(doc->extSubset, fullname, name);
@@ -3951,7 +3951,7 @@
 	
 	fullname = xmlBuildQName(elem->name, elem->ns->prefix, fn, 50);
 	if (fullname == NULL)
-	    return(0);
+	    return(NULL);
 	attrDecl = xmlGetDtdAttrDesc(doc->intSubset, fullname, name);
 	if ((attrDecl == NULL) && (doc->extSubset != NULL))
 	    attrDecl = xmlGetDtdAttrDesc(doc->extSubset, fullname, name);