avcodec/aom_film_grain: avoid duplicate indexes in ff_aom_parse_film_grain_sets()

Fixes: use after free
Fixes: 478301106/clusterfuzz-testcase-minimized-ffmpeg_dem_HEVC_fuzzer-6155792247226368

Found-by:  continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit ebb6ac1bc7)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/aom_film_grain.c

@@ -152,8 +152,9 @@ int ff_aom_parse_film_grain_sets(AVFilmGrainAFGS1Params *s,
         payload_4byte = get_bits1(gb);
         payload_size = get_bits(gb, payload_4byte ? 2 : 8);
         set_idx = get_bits(gb, 3);
+
         fgp = av_film_grain_params_alloc(&fgp_size);
-        if (!fgp)
+        if (!fgp || s->sets[set_idx])
             goto error;
         aom = &fgp->codec.aom;