Fix CTD on debugmsg with no Lua instance

[olanti-p]

Summary

SUMMARY: Bugfixes "Fix CTD on debugmsg with no Lua instance"

Purpose of change

Fix CTD when debugmsg tries to print Lua backtrace, but there is no Lua state available.

Describe the solution

Add nullptr check

Testing

Added fake debugmsg on game start, game no longer CTDs from it.

[scarf005]

maybe could we use std::optional?

[olanti-p]

Unfortunately std::optional<T> needs full definition of T, but I've deliberately hidden definition of cata::lua_state to avoid recompilations when switching between Lua and non-Lua builds.

The entire thing should be moved somewhere else to be honest, maybe with global accessor like get_map() or get_avatar(), it doesn't make sense to keep it in DynamicDataLoader after data has been loaded.