avcodec/evc_parse: Check num_remaining_tiles_in_slice_minus1

Fixes: out of array access
Fixes: 62467/clusterfuzz-testcase-minimized-ffmpeg_BSF_EVC_FRAME_MERGE_fuzzer-6092990982258688

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: "Dawid Kozinski/Multimedia (PLT) /SRPOL/Staff Engineer/Samsung Electronics" <d.kozinski@samsung.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/evc_parse.c

@@ -58,8 +58,12 @@ int ff_evc_parse_slice_header(GetBitContext *gb, EVCParserSliceHeader *sh,
         if (!sh->arbitrary_slice_flag)
             sh->last_tile_id = get_bits(gb, pps->tile_id_len_minus1 + 1);
         else {
-            sh->num_remaining_tiles_in_slice_minus1 = get_ue_golomb_long(gb);
-            num_tiles_in_slice = sh->num_remaining_tiles_in_slice_minus1 + 2;
+            unsigned num_remaining_tiles_in_slice_minus1 = get_ue_golomb_long(gb);
+            if (num_remaining_tiles_in_slice_minus1 > EVC_MAX_TILE_ROWS * EVC_MAX_TILE_COLUMNS - 2)
+                return AVERROR_INVALIDDATA;
+
+            num_tiles_in_slice = num_remaining_tiles_in_slice_minus1 + 2;
+            sh->num_remaining_tiles_in_slice_minus1 = num_remaining_tiles_in_slice_minus1;
             for (int i = 0; i < num_tiles_in_slice - 1; ++i)
                 sh->delta_tile_id_minus1[i] = get_ue_golomb_long(gb);
         }