Also, assign *_pp before unref-ing the old value. Calling
g_object_unref() on the old value, might invoke callbacks
that are out of control of nm_g_object_ref_set(). During
that time, the pointer should already be assigned the new value,
instead of having an intermediate %NULL value. In most cases,
this would of course not matter, but there is no need to let
anyone see an intermediate %NULL value for a moment.
Also, don't use typeof(**_pp), which would not work with opaque
types (like we commonly have).
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