gdbus-codegen: Fix segfault when remote property is invalidated

The 'key' variable is no longer valid outside the cycle, owned and
probably already freed by GVariant. This causes apps to segfault
when proxy is constructed and a property on remote d-bus service
changes (actually is invalidated). Looks like a typo anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=659070