Don't leak when marshalling GErrors to C

Python-land GLib.GErrors are supposed to have three attributes:
"message", "domain" and "code". If those attributes are missing, or they
have the wrong types, the C GError is filled in with a message
describing the error. The present-but-ill-typed code paths did not
DECREF the ill-typed values.

https://bugzilla.gnome.org/show_bug.cgi?id=666098