avdevice/pulse_audio_enc: use getter function for AVFrame.channels

This is required by the API/ABI for things outside libavutil
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 1e51af13)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavdevice/pulse_audio_enc.c

@@ -681,7 +681,7 @@ static int pulse_write_frame(AVFormatContext *h, int stream_index,
                AVERROR(EINVAL) : 0;
 
     pkt.data     = (*frame)->data[0];
-    pkt.size     = (*frame)->nb_samples * av_get_bytes_per_sample((*frame)->format) * (*frame)->channels;
+    pkt.size     = (*frame)->nb_samples * av_get_bytes_per_sample((*frame)->format) * av_frame_get_channels(*frame);
     pkt.dts      = (*frame)->pkt_dts;
     pkt.duration = av_frame_get_pkt_duration(*frame);
     return pulse_write_packet(h, &pkt);