avcodec/binkaudio: reset ch_offset on errors

Fixes: NULL pointer dereference
Fixes: 45955/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_BINKAUDIO_DCT_fuzzer-4842044192849920

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

libavcodec/binkaudio.c

@@ -329,6 +329,7 @@ again:
                      avctx->codec->id == AV_CODEC_ID_BINKAUDIO_DCT,
                      FFMIN(MAX_CHANNELS, s->channels - s->ch_offset), s->ch_offset)) {
         av_log(avctx, AV_LOG_ERROR, "Incomplete packet\n");
+        s->ch_offset = 0;
         return AVERROR_INVALIDDATA;
     }
     s->ch_offset += MAX_CHANNELS;
@@ -347,6 +348,7 @@ again:
 
     return 0;
 fail:
+    s->ch_offset = 0;
     av_packet_unref(s->pkt);
     return ret;
 }