Fix a crash in server_side_source_credentials_lookup_cb()
Fix a crash in server_side_source_credentials_lookup_cb()

A NULL 'error' variable dereference caused the crash. The change
also cleanups the code a bit, by removing incorrect runtime checks.

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