calendar-server: Fix double-free building variant

calendar-server: Fix double-free detection abort

app_notify_events_added uses an intermediate builder to construct an array that is then added to the main variant using g_variant_builder_add which should clear the intermediate, but doesn't due to the way it is passed: by value, rather than as a pointer.

This was debugged with the help of Eduardo Habkost, who believes it works on x86 due to big structs being passed as pointers.

Fixed: #3440 (closed)