avcodec/dxv: Check op_offset in both directions

Fixes: signed integer overflow: 61 + 2147483647 cannot be represented in type 'int'
Fixes: 15311/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DXV_fuzzer-5742552826773504

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

libavcodec/dxv.c

@@ -745,7 +745,7 @@ 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)
+    if (op_offset < 12 || op_offset - 12 > bytestream2_get_bytes_left(gb))
         return AVERROR_INVALIDDATA;
 
     dst = tex_data;