st7789: fix index out of bounds error

This commit adds the same check to the st7789 that also exists in the
st7735.

st7789/st7789.go

@@ -194,13 +194,14 @@ func (d *Device) FillRectangleWithBuffer(x, y, width, height int16, buffer []col
 	offset := int32(0)
 	for k > 0 {
 		for i := int32(0); i < d.batchLength; i++ {
-
+			if offset+i < int32(len(buffer)) {
 				c565 := RGBATo565(buffer[offset+i])
 				c1 := uint8(c565 >> 8)
 				c2 := uint8(c565)
 				data[i*2] = c1
 				data[i*2+1] = c2
 			}
+		}
 		if k >= d.batchLength {
 			d.Tx(data, false)
 		} else {