fix a memory leak in ws2812_buffer_shift

[Firenox89]

fix a memory leak in ws2812_buffer_shift by freeing the same amount of bytes we allocated before

Fixes #<GitHub-issue-number>.

Make sure all boxes are checked (add x inside the brackets) when you submit your contribution, remove this sentence before doing so.

• This PR is for the dev branch rather than for master.
• This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• I have thoroughly tested my contribution.
• The code changes are reflected in the documentation at docs/*.

Memory was allocated with luaM_malloc(L, sizeof(ws2812_buffer_shift_prepare) + shift_len);
but freed with luaM_free(L, prepare); this caused a leak by shift_len bytes per call.

The code I used to test with:

while (true) do
    buffer:shift(10, ws2812.SHIFT_CIRCULAR)
    ws2812.write(buffer)
    print(node.egc.meminfo())
    --coroutine.yield()
end

This caused a leak of 40 bytes per call before this commit, while after the numbers are stable.

[TerryE]

What this actually underlines in my mind is that we using Userdata for this type of usecase, then we can leave resource recovery to the Lua GC and this removes most paths for potential leakage.

[nwf]

ETA: Good find @Firenox89 and thank you for the PR!

I'm not opposed to this landing in dev now, since a fix is a fix, but I continue to hope that #2916 and my pixbuf replacement is ready for the next drop to master. https://github.com/nwf/nodemcu-firmware/blob/dev-active/app/modules/pixbuf.c already uses userdata for all its storage needs and eschews temporary allocations.