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>
(cherry picked from commit b20e61e0d52eef57cf5db55087b16e0b5207e730)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

block.c

@@ -1158,6 +1158,7 @@ int bdrv_open_image(BlockDriverState **pbs, const char *filename,
                        bdref_key);
             ret = -EINVAL;
         }
+        QDECREF(image_options);
         goto done;
     }