fixed panic in Set method

buffer.go

@@ -30,9 +30,17 @@ func NewBuffer(width, height int) *Buffer {
 	}
 }
 
+func (b *Buffer) x(x int) int {
+	return limit(x, 0, b.width-1)
+}
+
+func (b *Buffer) y(y int) int {
+	return limit(y, 0, b.height-1)
+}
+
 // Set sets the rune at position (x,y) to c
 func (b *Buffer) Set(x, y int, c rune) {
-	b.data[y][x] = c
+	b.data[b.y(y)][b.x(x)] = c
 }
 
 // Get returns the rune at position (x,y)