dix: NULL-protect AllocGlyphCursor()

Return error to the client if sourcefont turns out to be NULL for
whatever reason.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

dix/cursor.c

@@ -339,7 +339,7 @@ AllocGlyphCursor(Font source, unsigned sourceChar, Font mask, unsigned maskChar,
 
     rc = dixLookupResourceByType((void **) &sourcefont, source, X11_RESTYPE_FONT,
                                  client, DixUseAccess);
-    if (rc != Success) {
+    if ((rc != Success) || (!sourcefont)) {
         client->errorValue = source;
         return rc;
     }