guard against another NULL-deref in MRW
guard against another NULL-deref in MRW

darktable-org/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,anarsoul/rawspeed,klauspost/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,anarsoul/rawspeed,klauspost/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,anarsoul/rawspeed