s390/tty3270: ignore NUL characters

With 'TERM=vt220' zsh is sending several NUL characters with
the prompt to the tty. Both xterm and the linux drm console
seem to ignore them. Ignore them in tty3270 as well.
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

drivers/s390/char/con3270.c

@@ -1732,6 +1732,8 @@ static void tty3270_do_write(struct tty3270 *tp, struct tty_struct *tty,
 		}
 
 		switch (buf[i_msg]) {
+		case 0x00:
+			break;
 		case 0x07:		/* '\a' -- Alarm */
 			tp->wcc |= TW_PLUSALARM;
 			break;