shell bugfix bugfix

[utzoo!henry]

Bill Reeves's fix to shell mail checking, which I broadcast a couple
of days ago, inadvertently added a new bug.  Bill's mod fixed the shell
so it did mail checking properly in the presence of a mail system that
removes empty mailboxes;  the "fixed" shell misbehaves if your mail
system does NOT remove empty mailboxes.  The code which works right
either way is like this (about line 136 in main.c):

		THEN	IF mailnod.namval ANDF mailnod.namval[0] != '\0'
			    ANDF stat(mailnod.namval,&statb)>=0 
			    ANDF (statb.st_mtime != mailtime)
			THEN
				IF mailtime ANDF statb.st_size THEN prs(mailmsg) FI
				mailtime=statb.st_mtime;
			FI

This also incorporates another minor fix done here:  no mail checking
if the value of $MAIL is null.