allow limit to equal zero

It's reasonable for the number of resources to be set to zero. (The
limit could later be increased once resources are available, for
example.)

semaphore.go

@@ -63,8 +63,8 @@ type semaphore struct {
 
 // New initializes a new instance of the Semaphore, specifying the maximum number of concurrent entries.
 func New(limit int) Semaphore {
-	if limit <= 0 {
-		panic("semaphore limit must be greater than 0")
+	if limit < 0 {
+		panic("semaphore limit must not be negative")
 	}
 	broadcastCh := make(chan struct{})
 	return &semaphore{
@@ -189,8 +189,8 @@ func (s *semaphore) Release(n int) int {
 }
 
 func (s *semaphore) SetLimit(limit int) {
-	if limit <= 0 {
-		panic("semaphore limit must be greater than 0")
+	if limit < 0 {
+		panic("semaphore limit must not be negative")
 	}
 	for {
 		state := atomic.LoadUint64(&s.state)

semaphore_test.go

@@ -35,12 +35,8 @@ func TestNew(t *testing.T) {
 			checkLimitAndCount(t, sem, 1, 0)
 		},
 		func() {
-			defer func() {
-				if recover() == nil {
-					t.Error("Panic expected")
-				}
-			}()
-			_ = New(0)
+			sem := New(0)
+			checkLimitAndCount(t, sem, 0, 0)
 		},
 		func() {
 			defer func() {
@@ -359,18 +355,9 @@ func TestSemaphore_SetLimit(t *testing.T) {
 
 	sem.SetLimit(1)
 	checkLimitAndCount(t, sem, 1, 0)
-}
-
-func TestSemaphore_SetLimit_zero_limit_panic_expected(t *testing.T) {
-	sem := New(1)
-	checkLimitAndCount(t, sem, 1, 0)
 
-	defer func() {
-		if recover() == nil {
-			t.Error("Panic expected")
-		}
-	}()
 	sem.SetLimit(0)
+	checkLimitAndCount(t, sem, 0, 0)
 }
 
 func TestSemaphore_SetLimit_negative_limit_panic_expected(t *testing.T) {