avcodec/mpeg12dec: do not fail on invalid frame_rate_index

Fixes Ticket4073
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpeg12dec.c

@@ -2123,7 +2123,7 @@ static int mpeg1_decode_sequence(AVCodecContext *avctx,
     s->frame_rate_index = get_bits(&s->gb, 4);
     if (s->frame_rate_index == 0 || s->frame_rate_index > 13) {
         av_log(avctx, AV_LOG_WARNING, "frame_rate_index %d is invalid\n", s->frame_rate_index);
-        return -1;
+        s->frame_rate_index = 1;
     }
     s->bit_rate = get_bits(&s->gb, 18) * 400;
     if (get_bits1(&s->gb) == 0) { /* marker */