getmail() can only check the presence of an attribute flag currently

(not the absense), so we need to call it twice to find out if there are
any unread messages for the user.

src/sbbs3/readmail.cpp

@@ -800,7 +800,8 @@ void sbbs_t::readmail(uint usernumber, int which)
 			{
 				domsg = false;
 				if(!(lm_mode&LM_UNREAD)) {
-					if(!getmail(&cfg, usernumber, /* Sent: */FALSE, /* attr: */0)) {
+					if(getmail(&cfg, usernumber, /* Sent: */FALSE, /* attr: */0)
+						== getmail(&cfg, usernumber, /* Sent: */FALSE, /* attr: */MSG_READ)) {
 						bprintf(text[NoMailWaiting], "Un-read");
 						break;
 					}