avdevice/v4l2: only use average frame rate if set

It might be unset on some platforms with some drivers and some input

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

libavdevice/v4l2.c

@@ -447,6 +447,7 @@ static int init_convert_timestamp(AVFormatContext *ctx, int64_t ts)
         return 0;
     }
 #if HAVE_CLOCK_GETTIME && defined(CLOCK_MONOTONIC)
+    if (ctx->streams[0]->avg_frame_rate.num) {
         now = av_gettime_monotonic();
         if (s->ts_mode == V4L_TS_MONO2ABS ||
             (ts <= now + 1 * AV_TIME_BASE && ts >= now - 10 * AV_TIME_BASE)) {
@@ -460,6 +461,7 @@ static int init_convert_timestamp(AVFormatContext *ctx, int64_t ts)
             s->ts_mode = V4L_TS_CONVERT_READY;
             return 0;
         }
+    }
 #endif
     av_log(ctx, AV_LOG_ERROR, "Unknown timestamps\n");
     return AVERROR(EIO);
@@ -949,6 +951,7 @@ static int v4l2_read_header(AVFormatContext *s1)
         st->codec->codec_tag = MKTAG('Y', 'V', 'U', '9');
     st->codec->width = s->width;
     st->codec->height = s->height;
+    if (st->avg_frame_rate.den)
         st->codec->bit_rate = s->frame_size * av_q2d(st->avg_frame_rate) * 8;
 
     return 0;