avcodec/dxv: Check op_offset in dxv_decompress_cocg()

Fixes: signed integer overflow: -2147483648 - 12 cannot be represented in type 'int'
Fixes: 14732/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DXV_fuzzer-5735273129836544

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

libavcodec/dxv.c

@@ -745,6 +745,9 @@ static int dxv_decompress_cocg(DXVContext *ctx, GetByteContext *gb,
     int skip0, skip1, oi0 = 0, oi1 = 0;
     int ret, state0 = 0, state1 = 0;
 
+    if (op_offset < 12)
+        return AVERROR_INVALIDDATA;
+
     dst = tex_data;
     bytestream2_skip(gb, op_offset - 12);
     if (op_size0 > max_op_size0)