Fix type confusion in xmlValidateOneNamespace

Comment out code that casts xmlNsPtr to xmlAttrPtr. ID types on
namespace declarations make no practical sense anyway.

Fixes bug 780228.

Found with libFuzzer and ASan.

valid.c

@@ -4627,6 +4627,12 @@ xmlNodePtr elem, const xmlChar *prefix, xmlNsPtr ns, const xmlChar *value) {
 	}
     }
 
+    /*
+     * Casting ns to xmlAttrPtr is wrong. We'd need separate functions
+     * xmlAddID and xmlAddRef for namespace declarations, but it makes
+     * no practical sense to use ID types anyway.
+     */
+#if 0
     /* Validity Constraint: ID uniqueness */
     if (attrDecl->atype == XML_ATTRIBUTE_ID) {
         if (xmlAddID(ctxt, doc, value, (xmlAttrPtr) ns) == NULL)
@@ -4638,6 +4644,7 @@ xmlNodePtr elem, const xmlChar *prefix, xmlNsPtr ns, const xmlChar *value) {
         if (xmlAddRef(ctxt, doc, value, (xmlAttrPtr) ns) == NULL)
 	    ret = 0;
     }
+#endif
 
     /* Validity Constraint: Notation Attributes */
     if (attrDecl->atype == XML_ATTRIBUTE_NOTATION) {