tun: fix crash when ForceMTU is called after close

Close closes the events channel, resulting in a panic from send on
closed channel.

Reported-By: Brad Fitzpatrick <[email protected]>
Link: tailscale/tailscale#9555
Signed-off-by: James Tucker <[email protected]>

Author: raggi

tun/tun_windows.go

@@ -127,6 +127,9 @@ func (tun *NativeTun) MTU() (int, error) {
 
 // TODO: This is a temporary hack. We really need to be monitoring the interface in real time and adapting to MTU changes.
 func (tun *NativeTun) ForceMTU(mtu int) {
+	if tun.close.Load() {
+		return
+	}
 	update := tun.forcedMTU != mtu
 	tun.forcedMTU = mtu
 	if update {