cl/234850781 introduced a race condition in NIC.DeliverNetworkPacket
by failing to hold a lock. This change fixes this regressesion by acquiring
a read lock before iterating through n.endpoints, and then releasing the lock
once iteration is complete.
PiperOrigin-RevId: 235549770
Change-Id: Ib0133288be512d478cf759c3314dc95ec3205d4b