582906 XSD validating multiple imports of the same schema

* xmlschemas.c: When validating a schema that includes the same file
  that has no targetNamespace defined an internal erro was thrown,
  depending on the orig namespace that should be allowed though
* test/schemas/582906-* result/schemas/582906-*: 2 tests case, one
  where this is allowed, and one where this is forbidden
diff --git a/xmlschemas.c b/xmlschemas.c
index e621120..70fe3eb 100644
--- a/xmlschemas.c
+++ b/xmlschemas.c
@@ -11039,14 +11039,15 @@
 	    */
 	    isChameleon = 1;
 	    if (bucket->parsed &&
-		(bucket->targetNamespace != pctxt->targetNamespace)) {
-		/*
-		* This is a sanity check, I dunno yet if this can happen.
-		*/
-		PERROR_INT("xmlSchemaParseIncludeOrRedefine",
-		    "trying to use an already parsed schema for a "
-		    "different targetNamespace");
-		return(-1);
+		bucket->origTargetNamespace != NULL) {
+		xmlSchemaCustomErr(ACTXT_CAST pctxt,
+		    XML_SCHEMAP_SRC_INCLUDE,
+		    node, NULL,
+		    "The target namespace of the included/redefined schema "
+		    "'%s' has to be absent or the same as the "
+		    "including/redefining schema's target namespace",
+		    schemaLocation, NULL);
+		goto exit_error;
 	    }
 	    bucket->targetNamespace = pctxt->targetNamespace;
 	}