avcodec/mpegvideo_enc: Do not duplicate pictures on shifting

Fixes: out of array access
Fixes: 69098/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPEG2VIDEO_fuzzer-6107989688778752
Fixes: 69599/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPEG4_fuzzer-4848626296225792.fuzz

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

libavcodec/mpegvideo_enc.c

@@ -1304,6 +1304,8 @@ static int load_input_picture(MpegEncContext *s, const AVFrame *pic_arg)
     /* shift buffer entries */
     for (int i = flush_offset; i <= MAX_B_FRAMES; i++)
         s->input_picture[i - flush_offset] = s->input_picture[i];
+    for (int i = MAX_B_FRAMES + 1 - flush_offset; i <= MAX_B_FRAMES; i++)
+        s->input_picture[i] = NULL;
 
     s->input_picture[encoding_delay] = pic;