hashtable: Fix refcount of key/value in error case when marshalling
hashtable: Fix refcount of key/value in error case when marshalling

PyList_GET_ITEM only returns a borrowed reference. In the unlikely event
of an error converting the key or value, the reference would be
unintentionally decremented. This could cause random failures later on
(and was indeed causing the PyPy test to fail randomly later on).

GNOME/pygobject,GNOME/pygobject,GNOME/pygobject