avcodec/dxv: Subtract 12 earlier in dxv_decompress_cocg()

the data_start is after reading 12 bytes and if its subtracted
at the very end the intermediate might overflow

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit dd9e6d077e)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dxv.c

@@ -781,7 +781,7 @@ static int dxv_decompress_cocg(DXVContext *ctx, GetByteContext *gb,
             return ret;
     }
 
-    bytestream2_seek(gb, data_start + op_offset + skip0 + skip1 - 12, SEEK_SET);
+    bytestream2_seek(gb, data_start - 12 + op_offset + skip0 + skip1, SEEK_SET);
 
     return 0;
 }