animkeyframe: add some sanity checks
animkeyframe: add some sanity checks

Offsets checks prevent the time from being scaled up (easing shall be
defined only with t in the range [0;1]) and the boundary check prevents
a division by zero.

gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl