mirror: Release the dirty bitmap if mirror_start_job() fails

At the moment I don't see how to make this function fail after the
dirty bitmap has been created, but if that was possible then we would
hit the assert(QLIST_EMPTY(&bs->dirty_bitmaps)) in bdrv_close().

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/mirror.c

@@ -1649,6 +1649,9 @@ fail:
         g_free(s->replaces);
         blk_unref(s->target);
         bs_opaque->job = NULL;
+        if (s->dirty_bitmap) {
+            bdrv_release_dirty_bitmap(bs, s->dirty_bitmap);
+        }
         job_early_fail(&s->common.job);
     }