Panic in PM module in openonu-go adapter during Device Delete in Memory leak tests

openonu adapter crashes in PM module due to nil reference during device delete and some stale data is left on etcd (w.r.to PM data). As part of PrepareForGarbageCollection some pointers are set to nil to free up references for garbage collection by go runtime engine. This function is invoked during device delete. These nil pointers are referenced by PM FSM that is still preparing to shutdown.