fix: atomic.Value should be a concrete type to avoid panics (#13740)

Go's atomic.Value does not support `nil` type,
concrete type is necessary to avoid any panics with
the current implementation.

Also remove boolean to turn-off tracking of freezeCount.

cmd/handler-api.go

@@ -228,7 +228,7 @@ func (t *apiConfig) getRequestsPool() (chan struct{}, time.Duration) {
 func maxClients(f http.HandlerFunc) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		if val := globalServiceFreeze.Load(); val != nil {
-			if unlock, ok := val.(chan struct{}); ok {
+			if unlock, ok := val.(chan struct{}); ok && unlock != nil {
 				// Wait until unfrozen.
 				<-unlock
 			}