Don't attempt to parse origin addr from origin lines in NetMail msgs

Apparently, some versions or configurations of Mystic BBS will include an
Origin Line in FTN NetMail messages. That would've been okay for SBBSecho,
except these origin lines do not contain an FTN address (in parenthesis).

So, firstly: when importing NetMail messages (subnum=INVALID_SUB), don't
attempt to parse any Origin Lines. Secondly, fix up the Origin Line parse
code a bit better detect the case when no FTN address is actually present.