drivers/isdn: checkng length to be sure not memory overflow

sizeof (cmd.parm.cmsg.para) is 50 (MAX_CAPI_PARA_LEN).
  sizeof (cmd.parm) is 80+, but less than 100.
  strlen(msg) may be more than 80+ (Modem-Commandbuffer, less than 255).
    isdn_tty_send_msg is called by isdn_tty_parse_at
    the relative parameter is m->mdmcmd (atemu *m)
    the relative command may be "+M..."

  so need check the length to be sure not memory overflow.
    cmd.parm is a union, and need keep original valid buffer length no touch

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/i4l/isdn_tty.c

@@ -902,7 +902,9 @@ isdn_tty_send_msg(modem_info *info, atemu *m, char *msg)
 	int j;
 	int l;
 
-	l = strlen(msg);
+	l = min(strlen(msg), sizeof(cmd.parm) - sizeof(cmd.parm.cmsg)
+		+ sizeof(cmd.parm.cmsg.para) - 2);
+
 	if (!l) {
 		isdn_tty_modem_result(RESULT_ERROR, info);
 		return;