MRW: avoid crash in metadata handling
MRW: avoid crash in metadata handling

The TIFF metadata block may not have been found so check
for that before trying to dereference the pointer to look
for make and model.

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