Fix access violation in create_schematic()

[Treer]

Fixes #11533, schematics saved from y locations higher than 0 would cause an access violation if layer probabilities were specified

See #11533 for more details

To do

This PR is Ready for Review.

How to test

• Attached is a mod that reproduces the error, run /nocrashy chat command for the mod to save a schematic without crashing, and /crashy for it to cause an access violation.
• Operations performed before create_schematic() is called can affect whether Minetest will crash, so different lua code doesn't always reproduce it, and this may also be the case across different machines.
• Testing that layer probabilities are now correctly applied (in addition to not crashing) requires serializing the output of create_schematic() into lua and inspecting the yslice_prob table. The one i tried looked good to me.
• I've not checked whether l_create_schematic() sanitizes the ypos it receives from lua before using it as an index. (I didn't notice any, but also don't know what the policy is)

[sfan5]

Looked at related code, looks correct.