Mail: correctly track the number of messages (thanks igor)

We forgot to update the message count when deleting
messages in mail, meaning we could access trailing
messages that had been freed.

sys/src/cmd/upas/Mail/mbox.c

@@ -715,6 +715,7 @@ mbflush(char **, int)
 		else
 			mbox.mesg[j++] = m;
 	}
+	mbox.nmesg = j;
 		
 	close(fd);
 	fprint(mbox.ctl, "clean\n");