getusrsub() returned the completely wrong sub-board number. It assumed that
getusrgrp() returned a 0-based index when in fact it returns a 1-based group
number. This bug was only visible when using certain @-codes (e.g. "SN",
"SL", "SR", and "SMB_SUB_NUM") when a sub-board was open via Synchronet
internal function (i.e. not via JS MsgBase class). If the current group was the
last group visible to the user, this would cause a crash.