block: Change bitmap truncate conditional to assertion

This is an artifact of an older version that had both all-bitmap and
single-bitmap truncate functions, and some info got lost in the shuffle.

Bitmaps can only be frozen during a backup operation, and a backup
operation should prevent a resize operation, so just assert that this
cannot happen.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
