avcodec/pdvdec: Check previous frame before using it

Fixes: load of null pointer of type 'uint8_t' (aka 'unsigned char')
Fixes: 58256/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PDV_fuzzer-4776436325285888

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

libavcodec/pdvdec.c

@@ -93,7 +93,7 @@ static int decode_frame(AVCodecContext *avctx, AVFrame *frame,
             return AVERROR_INVALIDDATA;
         }
 
-        if (!(avpkt->flags & AV_PKT_FLAG_KEY)) {
+        if (prev && !(avpkt->flags & AV_PKT_FLAG_KEY)) {
             for (int j = 0; j < (avctx->width + 7) >> 3; j++)
                 dst[j] ^= prev[j];
             prev += prev_frame->linesize[0];