GSettings: two memory use fixes

First, correct a rather dubious case of accessing a GSettingsSchemaKey
after clearing it.  This was technically okay because only the key name
was accessed (and it is not owned by the struct) but it looks very
wrong.

Second, have g_settings_backend_write() sink the passed in GVariant*.
Not all backends get this right, and I'm starting to like the pattern of
virtual function wrappers being responsible for sinking the parameters
that they are documented as consuming.