qcow2: Silence false warning

Some gcc versions seem not to be able to figure out that the switch
statement covers all possible values and that c is therefore always
initialised. Add a default branch for them.
Reported-by: Nmalc <av1474@comtv.ru>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: Nmalc <av1474@comtv.ru>

block/qcow2-cluster.c

@@ -471,6 +471,8 @@ int qcow2_get_cluster_offset(BlockDriverState *bs, uint64_t offset,
                 QCOW_OFLAG_COMPRESSED | QCOW_OFLAG_ZERO);
         *cluster_offset &= L2E_OFFSET_MASK;
         break;
+    default:
+        abort();
     }
 
     qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);