Fix Splice crash due to Core issue, #3269.

@helgemathee Fix SpliceMaya crash. The issue was due to Splice creating the new Client object when loading a scene before destroying the previous Client object and a bug in the Core was causing extension shared libraries to be incorrectly unloaded when the original Client object was finally closed. This was then causing crashes when calling into extension code. I've filed an issue to fix the underlying Core problem for an upcoming release and have fixed Splice to properly destroy the first Client object before creating the new one.