It's possible to use savemsg() (from JS MsgBase.save_msg()) to add a message
to a message base that's not a sub-board (not configured in SCFG->Message
Areas and not the "mail" base), but in that case, savemsg() would dereference
an invalid sub in the scfg->sub array and crash. So use is_valid_subum()
to insure the subnum is a valid sub before using as an index.