Fix retain cycle between NWPathMonitor and PacketTunnelProvider

See: https://www.marisibrothers.com/2017/04/memory-leak-in-swift-assigning-function.html

Signed-off-by: Andrej Mihajlov <and@mullvad.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

WireGuard/WireGuardNetworkExtension/PacketTunnelProvider.swift

@@ -52,7 +52,9 @@ class PacketTunnelProvider: NEPacketTunnelProvider {
                 startTunnelCompletionHandler(PacketTunnelProviderError.couldNotSetNetworkSettings)
             } else {
                 self.networkMonitor = NWPathMonitor()
-                self.networkMonitor!.pathUpdateHandler = self.pathUpdate
+                self.networkMonitor!.pathUpdateHandler = { [weak self] path in
+                    self?.pathUpdate(path: path)
+                }
                 self.networkMonitor!.start(queue: DispatchQueue(label: "NetworkMonitor"))
 
                 let fileDescriptor = (self.packetFlow.value(forKeyPath: "socket.fileDescriptor") as? Int32) ?? -1