SPI DMA: avoid 0-byte transfers!

Author: PaintYourDragon

Adafruit_SPITFT.cpp

@@ -511,6 +511,7 @@ void Adafruit_SPITFT::writeColor(uint16_t color, uint32_t len) {
     if(_sclk < 0) { // Using hardware SPI
 
 #ifdef USE_SPI_DMA
+        if(!len) return; // DO NOT ANGER DMA WITH 0-BYTE TRANSFERS!
 
         int i, d, numDescriptors;
         if(hi == lo) { // If high & low bytes are same...