ffmpeg: use isatty() before messing with the terminal state

This fixes terminal messup in case of crashes (like in make fate)
Reviewed-by: François Revol <revol@free.fr>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -1217,7 +1217,9 @@ static void term_init(void)
 #if HAVE_TERMIOS_H
     if(!run_as_daemon){
         struct termios tty;
-
+#if HAVE_ISATTY
+        if(isatty(0) && isatty(2))
+#endif
         if (tcgetattr (0, &tty) == 0) {
             oldtty = tty;
             restore_tty = 1;