GDBusProxy: on_properties_changed initialize some variables

GDBusProxy: on_properties_changed initialize some variables

This avoids calling g_variant_unref and g_free on uninitialized memory
if PropertiesChanged is received in the creating thread's thread-default
main context's thread, at the same time as releasing the last ref in
another thread. This would result in "goto out" before the variables
freed after that label had been initialized to NULL.

Based on a patch by Simon McVittie, bug 656282