block: Fix use after free error in bdrv_open_inherit()

When a block device is opened with BDRV_O_SNAPSHOT and the
bdrv_append_temp_snapshot() call fails then the error code path tries
to unref the already destroyed 'options' QDict.

This can be reproduced easily by setting TMPDIR to a location where
the QEMU process can't write:

   $ TMPDIR=/nonexistent $QEMU -drive driver=null-co,snapshot=on

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -2792,6 +2792,7 @@ static BlockDriverState *bdrv_open_inherit(const char *filename,
     bdrv_parent_cb_change_media(bs, true);
 
     qobject_unref(options);
+    options = NULL;
 
     /* For snapshot=on, create a temporary qcow2 overlay. bs points to the
      * temporary snapshot afterwards. */