Don't truncate a user's record if the default download protocol or gender are '\0'

A blank download protocol field in a user.dat, when parsed, sets the 'prot' field
of user_t to 0. When writing the record back to the user.dat, this would prematurely
truncate all other fields off the user record (since strings in C are NUL terminated
and we're using sprintf() to format the record and %c specifier for that field).

The fix is to write a ' ' character instead of '\0' if the user_t.prot is '\0'.
As part of this fix, I'm writing a '?' if a user_t.sex is '\0' (not sure if this
is actually possible, but just as insurance). Those are the only 2 single-character
user properties/fields today.

Bug reported/debugged by Al of The Rusty Mailbox (1:153/757.2) - thank you!

src/sbbs3/userdat.c

@@ -687,7 +687,7 @@ BOOL format_userdat(scfg_t* cfg, user_t* user, char userdat[])
 		,user->zipcode
 		,user->phone
 		,user->birth
-		,user->sex
+		,user->sex ? user->sex : '?'
 		,user->comment
 		,user->modem
 		,user->misc
@@ -698,7 +698,7 @@ BOOL format_userdat(scfg_t* cfg, user_t* user, char userdat[])
 		,user->xedit && user->xedit <= cfg->total_xedits ? cfg->xedit[user->xedit - 1]->code : ""
 		,user->shell < cfg->total_shells ? cfg->shell[user->shell]->code : ""
 		,user->tmpext
-		,user->prot
+		,user->prot ? user->prot : ' '
 		,user->cursub
 		,user->curdir
 		,user->curxtrn