block: Fix use after free error in bdrv_open_inherit()

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>