avcodec/sbcdec: Initialize number of channels

Fixes: out of array access
Fixes: 17609/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SBC_fuzzer-5758729319874560

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Reviewed-by: Moritz Barsnick <barsnick@gmx.net>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/sbcdec.c

@@ -348,6 +348,7 @@ static int sbc_decode_frame(AVCodecContext *avctx,
     if (frame_length <= 0)
         return frame_length;
 
+    avctx->channels =
     frame->channels = sbc->frame.channels;
     frame->format = AV_SAMPLE_FMT_S16P;
     frame->nb_samples = sbc->frame.blocks * sbc->frame.subbands;