client/systray: add missing deferred unlock for httpCache mutex

Updates #cleanup

Change-Id: Ia101a4a3005adb9118051b3416f5a64a4a45987d
Signed-off-by: Will Norris <will@tailscale.com>

client/systray/systray.go

@@ -372,6 +372,7 @@ func setRemoteIcon(menu *systray.MenuItem, urlStr string) {
 	}
 
 	cacheMu.Lock()
+	defer cacheMu.Unlock()
 	b, ok := httpCache[urlStr]
 	if !ok {
 		resp, err := http.Get(urlStr)
@@ -395,7 +396,6 @@ func setRemoteIcon(menu *systray.MenuItem, urlStr string) {
 			resp.Body.Close()
 		}
 	}
-	cacheMu.Unlock()
 
 	if len(b) > 0 {
 		menu.SetIcon(b)