check before unref, this will fail in BadCreds test

src/core/surface/channel.c

@@ -229,7 +229,9 @@ static void destroy_channel(void *p, int ok) {
     registered_call *rc = channel->registered_calls;
     channel->registered_calls = rc->next;
     GRPC_MDELEM_UNREF(rc->path);
-    GRPC_MDELEM_UNREF(rc->authority);
+    if (rc->authority) {
+      GRPC_MDELEM_UNREF(rc->authority);
+    }
     gpr_free(rc);
   }
   grpc_mdctx_unref(channel->metadata_context);