Safer engine functions

[BR-]

common.map_set will crash if called on an improper map. It should instead throw a Lua error. I suspect there are many other functions like this.

[rakiru]

Something in the networking will crash too. I think net_unpack may crash if the buffer isn't long enough. net_pack should probably be checked too (then again, most of them should probably at least get a look at them).

[LeComm]

Especially the map functions are REALLY crashy, partly because of all the "dirty" pillar/column stuff. I wonder why iceball uses the vxl format instead of something like in PySnip.

[iamgreaser]

Because it's faster to look geometry up with and it uses less memory.