network/arp: Add assertion that the cache is empty upon uninit.

May help with diagnosing #19185.

src/add-ons/kernel/network/datalink_protocols/arp/arp.cpp

@@ -960,6 +960,7 @@ arp_init()
 static status_t
 arp_uninit()
 {
+	ASSERT(sCache->IsEmpty());
 	unregister_generic_syscall(ARP_SYSCALLS, 1);
 	return B_OK;
 }