Bounds check for ws2812.buffer:replace does not work

[HHHartmann]

I found this error while writing tests for this module.
So the effort was worth it right away.

Expected behavior

buffer:replace with invalid replace position throw an error.

Actual behavior

Does not abort and possibly corrupts memory.

Test code

strip_buffer = ws2812.newBuffer(5, 3)
strip_buffer:replace(string.char(3,255,165,33,0,244,12,87,255), -7)
strip_buffer:dump()

the last commad reboots the mcu after some seconds

NodeMCU version

NodeMCU 3.0.0.0
branch: dev
commit: 0b90495

Hardware

standard dev board

[HHHartmann]

Need to fix

static ptrdiff_t posrelat (ptrdiff_t pos, size_t len)

and adapt all occurrences if needed