block: Simplify qmp_block_resize() error paths

The only thing that happens after the 'out:' label is blk_unref(blk).
However, blk = NULL in all of the error cases, so instead of jumping to
'out:', we can just return directly.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20201203172311.68232-2-kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>