qemu-img rebase: Fix segfault if backing file can't be opened

bdrv_delete must not be called for a NULL BlockDriverState.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

qemu-img.c

@@ -1411,8 +1411,12 @@ out:
     qemu_progress_end();
     /* Cleanup */
     if (!unsafe) {
-        bdrv_delete(bs_old_backing);
-        bdrv_delete(bs_new_backing);
+        if (bs_old_backing != NULL) {
+            bdrv_delete(bs_old_backing);
+        }
+        if (bs_new_backing != NULL) {
+            bdrv_delete(bs_new_backing);
+        }
     }
 
     bdrv_delete(bs);