fixed panic when writing to zero-sized buffer

buffer.go

@@ -42,7 +42,9 @@ func (b *Buffer[T]) y(y int) int {
 
 // Set sets the value at position (x,y) to c
 func (b *Buffer[T]) Set(x, y int, v T) {
+	if b.width >= 0 && b.height >= 0 {
 		b.data[b.y(y)][b.x(x)] = v
+	}
 }
 
 // Get returns the value at position (x,y)

buffer_test.go

@@ -17,3 +17,8 @@ func TestSub(t *testing.T) {
 	fmt.Println(strings.Repeat("-", 10))
 	fmt.Println(s)
 }
+
+func TestSet(t *testing.T) {
+	b := NewBuffer(10, 10, ' ')
+	b.Set(11, 0, 'a')
+}