It typically only makes sense to do this on a stack-allocated VariantDict if you
want to abort building the value part-way through. This function need not be called if you call
end and it also doesn't need to be called on dicts allocated with
g_variant_dict_unref for that).
It is valid to call this function on either an initialised VariantDict or one that was previously cleared by an earlier call to clear but it is not valid to call this function on uninitialised memory.