more cleanups based on sparse reports, added "make sparse" Daniel

* Makefile.am globals.c parserInternals.c xmlreader.c xmlunicode.c
  xmlwriter.c: more cleanups based on sparse reports, added
  "make sparse"
Daniel
diff --git a/xmlreader.c b/xmlreader.c
index fecc547..f3b5520 100644
--- a/xmlreader.c
+++ b/xmlreader.c
@@ -598,14 +598,14 @@
     xmlNodePtr ret;
 
     if (reader->entNr <= 0)
-        return (0);
+        return (NULL);
     reader->entNr--;
     if (reader->entNr > 0)
         reader->ent = reader->entTab[reader->entNr - 1];
     else
         reader->ent = NULL;
     ret = reader->entTab[reader->entNr];
-    reader->entTab[reader->entNr] = 0;
+    reader->entTab[reader->entNr] = NULL;
     return (ret);
 }
 
@@ -2172,7 +2172,7 @@
 	if ((reader->ctxt->vctxt.vstateTab != NULL) &&
 	    (reader->ctxt->vctxt.vstateMax > 0)){
 	    xmlFree(reader->ctxt->vctxt.vstateTab);
-	    reader->ctxt->vctxt.vstateTab = 0;
+	    reader->ctxt->vctxt.vstateTab = NULL;
 	    reader->ctxt->vctxt.vstateMax = 0;
 	}
 	if (reader->allocs & XML_TEXTREADER_CTXT)