block: Plug memory leak on brv_open_image() error path

Introduced in commit da557a.  Spotted by Coverity.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -1228,6 +1228,7 @@ int bdrv_open_image(BlockDriverState **pbs, const char *filename,
                        bdref_key);
             ret = -EINVAL;
         }
+        QDECREF(image_options);
         goto done;
     }