avconv: check output stream recording time before each frame returned from filters

There may be multiple frames returned, so with just one check we can
write more than requested to the output.

avconv.c

@@ -1668,7 +1668,7 @@ static int poll_filters(void)
         OutputFile    *of = output_files[ost->file_index];
         int ret = 0;
 
-        if (!ost->filter || ost->is_past_recording_time)
+        if (!ost->filter)
             continue;
 
         if (!ost->filtered_frame && !(ost->filtered_frame = avcodec_alloc_frame())) {
@@ -1677,7 +1677,7 @@ static int poll_filters(void)
             avcodec_get_frame_defaults(ost->filtered_frame);
         filtered_frame = ost->filtered_frame;
 
-        while (ret >= 0) {
+        while (ret >= 0 && !ost->is_past_recording_time) {
             if (ost->enc->type == AVMEDIA_TYPE_AUDIO &&
                 !(ost->enc->capabilities & CODEC_CAP_VARIABLE_FRAME_SIZE))
                 ret = av_buffersink_read_samples(ost->filter->filter, &picref,