pcmdec: change default of channels parameter to 1

This was previously implicitly done in ff_raw_read_header().
Fixes fate tests with pcm input.

libavformat/pcmdec.c

@@ -83,7 +83,7 @@ static int raw_read_packet(AVFormatContext *s, AVPacket *pkt)
 
 static const AVOption pcm_options[] = {
     { "sample_rate", "", offsetof(RawAudioDemuxerContext, sample_rate), AV_OPT_TYPE_INT, {.i64 = 0}, 0, INT_MAX, AV_OPT_FLAG_DECODING_PARAM },
-    { "channels",    "", offsetof(RawAudioDemuxerContext, channels),    AV_OPT_TYPE_INT, {.i64 = 0}, 0, INT_MAX, AV_OPT_FLAG_DECODING_PARAM },
+    { "channels",    "", offsetof(RawAudioDemuxerContext, channels),    AV_OPT_TYPE_INT, {.i64 = 1}, 0, INT_MAX, AV_OPT_FLAG_DECODING_PARAM },
     { NULL },
 };