luks: Turn another invalid assertion into check

Commit e39e959e fixed an invalid assertion in the .bdrv_length
implementation, but left a similar assertion in place for
.bdrv_truncate. Instead of crashing when the user requests a too large
image size, fail gracefully.

A file size of exactly INT64_MAX caused failure before, but is actually
legal.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
1 file changed