block/qapi: Clear err for further error

Since a5002d53 (block/qapi: allow best-effort query) we don't return at
this error, however err must be cleared before passing to
bdrv_query_snapshot_info_list below, as required by error API.
Signed-off-by: NFam Zheng <famz@redhat.com>
Message-id: 1450779107-26765-1-git-send-email-famz@redhat.com
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>

block/qapi.c

@@ -250,6 +250,7 @@ void bdrv_query_image_info(BlockDriverState *bs,
             g_free(backing_filename2);
             backing_filename2 = NULL;
             error_free(err);
+            err = NULL;
         }
 
         /* Always report the full_backing_filename if present, even if it's the