Fix indices in unrolled memcpy

While investigating possible causes for issue #15 I noticed that there was an error copying the last two bytes of each packet. This seems to be a buffer overrun introduced in commit dd4073bfa1c069c298cd66028c16eaff17ce9014 "Avoid strict aliasing warning from gcc arm 4.7 - it's a silly warning."
I was hoping this might fix the issue... but it does not.