migration: Drop reference to QIOChannel if file seeking fails

We forgot to drop the reference to the QIOChannel in the error path of
the offset adjustment. Do it now.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

migration/file.c

@@ -94,6 +94,7 @@ void file_start_outgoing_migration(MigrationState *s,
 
     ioc = QIO_CHANNEL(fioc);
     if (offset && qio_channel_io_seek(ioc, offset, SEEK_SET, errp) < 0) {
+        object_unref(OBJECT(fioc));
         return;
     }
     qio_channel_set_name(ioc, "migration-file-outgoing");