Fix Root IFD is leaked when merging (all content is transferred, but the root IFD itself is not).
Fix Root IFD is leaked when merging (all content is transferred, but the root IFD itself is not).

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