Avoid crash in e_book_backend_file_load_revision()
Avoid crash in e_book_backend_file_load_revision()

There could happen a crash in e_book_backend_file_load_revision()
due to NULL 'error' when an application did a quick open/close
operations, in which case a backend was reused, with already set
priv::revision, and the call failed on a runtime constraint, but
didn't set the 'error', which led to the crash.

Distrotech/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server,Distrotech/evolution-data-server,tintou/evolution-data-server,Distrotech/evolution-data-server,Distrotech/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,Distrotech/evolution-data-server