avcodec/osq: Implement flush()

Fixes: out of array access
Fixes: 62164/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_OSQ_fuzzer-6227491892887552
Fixes: 62164/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_OSQ_fuzzer-6268561729126400
Fixes: 62164/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_OSQ_fuzzer-6414805046788096
Fixes: 62164/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_OSQ_fuzzer-6538151088488448
Fixes: 62164/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_OSQ_fuzzer-6608131540779008

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

libavcodec/osq.c

@@ -61,6 +61,14 @@ typedef struct OSQContext {
     int pkt_offset;
 } OSQContext;
 
+static void osq_flush(AVCodecContext *avctx)
+{
+    OSQContext *s = avctx->priv_data;
+
+    s->bitstream_size = 0;
+    s->pkt_offset = 0;
+}
+
 static av_cold int osq_close(AVCodecContext *avctx)
 {
     OSQContext *s = avctx->priv_data;
@@ -478,4 +486,5 @@ const FFCodec ff_osq_decoder = {
                                                         AV_SAMPLE_FMT_S16P,
                                                         AV_SAMPLE_FMT_S32P,
                                                         AV_SAMPLE_FMT_NONE },
+    .flush            = osq_flush,
 };