shifting more pixels than available for ws2812.buffer:shift reboots

[HHHartmann]

I found this error while writing tests for this module.
So the effort was worth it right away.
This Bug was introduced a while ago, so if we had had tests then it would not be there.

Expected behavior
buffer:shift with more pixels than selected(available) should throw an error.

Actual behavior
Reboots.

Test code

buffer = ws2812.newBuffer(4, 4)
buffer:shift(-10)

the last commad reboots the mcu

NodeMCU version
NodeMCU 3.0.0.0
branch: dev
commit: 0b90495

Hardware
standard dev board