qcow2: Fix error handling in qcow2_alloc_cluster_offset

If do_alloc_cluster_offset() fails, the error handling code tried to
remove the request from the in-flight queue, to which it wasn't added
yet, resulting in a NULL pointer dereference.

m->nb_clusters really only becomes != 0 when the request is in the list.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2-cluster.c

@@ -931,7 +931,7 @@ again:
 fail:
     qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
 fail_put:
-    if (nb_clusters > 0) {
+    if (m->nb_clusters > 0) {
         QLIST_REMOVE(m, next_in_flight);
     }
     return ret;