avcodec/shorten: clear padding

Fixes: use-of-uninitialized-value
Fixes: 70854/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SHORTEN_fuzzer-5533480570650624

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

libavcodec/shorten.c

@@ -563,6 +563,7 @@ static int shorten_decode_frame(AVCodecContext *avctx, AVFrame *frame,
     buf               = &s->bitstream[s->bitstream_index];
     buf_size         += s->bitstream_size;
     s->bitstream_size = buf_size;
+    memset(buf + buf_size, 0, AV_INPUT_BUFFER_PADDING_SIZE);
 
     /* do not decode until buffer has at least max_framesize bytes or
      * the end of the file has been reached */