Fix seg fault if popping from empty stack (L-system trees)

See: https://github.com/minetest/minetest/issues/1525

Background
Wuzzy2: If you attempt to spawn a L-system tree with minetest.spawn_tree, you can make Minetest crash if it is attempted to pop an empty stack.

ShadowNinja: This shouldn't cause a segmentation fault, but it should throw a Lua error

Commit Description
This commit throws a Lua error instead of causing a segmentation fault. The server will still "crash" but will include a Lua backtrace.