Blender Git Loki

Fix T49369: Blender crashes/closes down application at alembic export of
any object

There were a couple of crashes caused by stupid typos in
rB631af9f930d2fd2c76751204ff22239aa95f761d and
rB78ea06fea4a74181c25254ed72d50d8a743b6954, as well as a shamefull lack
of 'testing before committing' which only affect exporting.

One crash was due to using RNA_boolean_get instead of RNA_enum_get, the
other one was a tricky case of order of deletion happening in the
destructors of AbcExporter and ArchiveWriter.

Should not affect RC or release.