avcodec/iff: Fix mask_buf / mask_palbuf leak

Fixes: 15372/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_IFF_ILBM_fuzzer-5708881759567872

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

libavcodec/iff.c

@@ -371,6 +371,8 @@ static av_cold int decode_end(AVCodecContext *avctx)
     av_freep(&s->planebuf);
     av_freep(&s->ham_buf);
     av_freep(&s->ham_palbuf);
+    av_freep(&s->mask_buf);
+    av_freep(&s->mask_palbuf);
     av_freep(&s->video[0]);
     av_freep(&s->video[1]);
     av_freep(&s->pal);