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.
moving where we NULL the *leds pointer
Merge pull request #13 from TwP/set-freed-pointers-to-null

Set freed pointers to NULL and free device in cleanup.  This allows multiple calls to the cleanup function without segfault/corruption.