ffmpeg: check tcgetattr result.

This fixes parallel FATE (make fate -j4) failing under valgrind with:
 Syscall param ioctl(TCSET{S,SW,SF}) points to uninitialised byte(s)
    at 0x5D98B23: tcsetattr (tcsetattr.c:88)
    by 0x43D66C: term_init (ffmpeg.c:734)
    by 0x43CD8D: main (ffmpeg.c:5071)
  Address 0x7fefffdd0 is on thread 1's stack
  Uninitialised value was created by a stack allocation
    at 0x43D5B0: term_init (ffmpeg.c:716)

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>