avcodec/apac: reset buffer on error

Fixes: repeatly parsing the same data after each 1 byte packet
Fixes: Timeout
Fixes: 51943/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_APAC_fuzzer-5779018251370496

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

libavcodec/apac.c

@@ -191,6 +191,8 @@ static int apac_decode(AVCodecContext *avctx, AVFrame *frame,
             if (c->bit_length < 0 ||
                 c->bit_length > 17) {
                 c->bit_length = avctx->bits_per_coded_sample;
+                s->bitstream_index = 0;
+                s->bitstream_size  = 0;
                 return AVERROR_INVALIDDATA;
             }