Don't write junk to unused bytes of downfil and motdfil

These files don't use all members of struct telstr.  turn() neglected
to initialized the unused ones and struct telstr's holes.  Fix that.

src/lib/commands/turn.c

@@ -107,6 +107,7 @@ turn(void)
     if (msgfilepath == downfil)
 	pr("Logins disabled.\n");
 
+    memset(&tgm, 0, sizeof(tgm));
     if ((fwrite(&tgm, sizeof(tgm), 1, fptr) != 1) ||
 	(fwrite(msgbuf, tgm.tel_length, 1, fptr) != 1)) {
 	fclose(fptr);