This commit introduces pointer checks to various functions;
so that entity_* will no longer crash or produce weird results
if passed a Node pointer, etc.
The checks are disabled by default, but can be enabled in
ScriptInterface.cpp.
Fixed possible crashes in a few more functions due to missing NULL-checks.
There was a "feature" in the single Lua state that it would keep globals
intact until the game was quit. That made any globals from mods "leak"
into the game or other mods. Now it resets the Lua state when a mod
is loaded or closed.
