VC5Decompressor::decodeLargeCodeblock(): assert that we don't overwrite existing band
VC5Decompressor::decodeLargeCodeblock(): assert that we don't overwrite existing band

It doesn't really make sense to first decode it, and then overwrite it.

darktable-org/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed