avformat/img2dec: Clear padding data after EOF

Fixes: use-of-uninitialized-value
Fixes: 70852/clusterfuzz-testcase-minimized-ffmpeg_IO_DEMUXER_fuzzer-5179190066872320

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Kacper Michajlow <kasper93@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/img2dec.c

@@ -563,6 +563,7 @@ int ff_img_read_packet(AVFormatContext *s1, AVPacket *pkt)
         }
         goto fail;
     } else {
+        memset(pkt->data + pkt->size, 0, AV_INPUT_BUFFER_PADDING_SIZE);
         s->img_count++;
         s->img_number++;
         s->pts++;