couple of memory fixes from Mark Vakoc reported by Purify on Windows.

* threads.c uri.c: couple of memory fixes from Mark Vakoc reported
  by Purify on Windows.
Daniel

threads.c

@@ -369,6 +369,8 @@ xmlRMutexUnlock(xmlRMutexPtr tok ATTRIBUTE_UNUSED)
 static void
 xmlFreeGlobalState(void *state)
 {
+	/* free any memory allocated in the thread's xmlLastError */
+	xmlResetLastError();
     free(state);
 }