avfilter/buffersrc: check for valid sample rate

A sample rate <= 0 is invalid.

Fixes an assert in ffmpeg_enc.c that assumed a valid sample rate would be set.
Fixes ticket #11385.

Signed-off-by: James Almer <jamrial@gmail.com>

libavfilter/buffersrc.c

@@ -421,6 +421,11 @@ static av_cold int init_audio(AVFilterContext *ctx)
         av_channel_layout_describe(&s->ch_layout, buf, sizeof(buf));
     }
 
+    if (s->sample_rate <= 0) {
+        av_log(ctx, AV_LOG_ERROR, "Sample rate not set\n");
+        return AVERROR(EINVAL);
+    }
+
     if (!s->time_base.num)
         s->time_base = (AVRational){1, s->sample_rate};