Fix bug that crash with negative width/height

Author: mattn

sixel.go

@@ -37,10 +37,13 @@ const (
 func (e *Encoder) Encode(img image.Image) error {
 	nc := 255 // (>= 2, 8bit, index 0 is reserved for transparent key color)
 	width, height := img.Bounds().Dx(), img.Bounds().Dy()
-	if e.Width != 0 {
+	if width == 0 || height == 0 {
+		return nil
+	}
+	if e.Width > 0 {
 		width = e.Width
 	}
-	if e.Height != 0 {
+	if e.Height > 0 {
 		height = e.Height
 	}