wgengine/netlog: fix send to closed channel in test

Fixes #17922

Change-Id: I2cd600b0ecda389079f2004985ac9a25ffbbfdd1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/netlog/netlog_test.go

@@ -182,6 +182,7 @@ func TestUpdateRace(t *testing.T) {
 	group.Wait()
 	logger.mu.Lock()
 	close(logger.recordsChan)
+	logger.recordsChan = nil
 	logger.mu.Unlock()
 }