Set pointers to NULL after freeing memory.
Set pointers to NULL after freeing memory.

Calling `ws2811_cleanup` multiple times will result in a segmentation fault.
Setting freed poitners to NULL prevents this from happening.

jgarff/rpi_ws281x,plan44/rpi_ws281x,piers7/rpi_ws281x,weidnerm/pi-ws2812,euang/rpi_ws281x,beyondscreen/rpi_ws281x,piers7/rpi_ws281x,jgarff/rpi_ws281x,Margravegit/rainbowraceway,euang/rpi_ws281x,richardghirst/rpi_ws281x,mludvig/rpi_ws281x,Thor77/rpi_ws281x,beyondscreen/rpi_ws281x,Thor77/rpi_ws281x,plan44/rpi_ws281x,thejpster/rpi_ws281x,raspberry-node/rpi_ws281x,plan44/rpi_ws281x,tykune21/rpi_ws281x,willowcheng/makerthings-neopixels,weidnerm/pi-ws2812,mludvig/rpi_ws281x,willowcheng/makerthings-neopixels,tykune21/rpi_ws281x,euang/rpi_ws281x,willowcheng/makerthings-neopixels,piers7/rpi_ws281x,weidnerm/pi-ws2812,richardghirst/rpi_ws281x,mludvig/rpi_ws281x,raspberry-node/rpi_ws281x,richardghirst/rpi_ws281x,thejpster/rpi_ws281x,Margravegit/rainbowraceway,Margravegit/rainbowraceway,raspberry-node/rpi_ws281x,tykune21/rpi_ws281x,thejpster/rpi_ws281x,piers7/rpi_ws281x,weidnerm/pi-ws2812,weidnerm/pi-ws2812,Thor77/rpi_ws281x,jgarff/rpi_ws281x