Fix false positive no return warnings in msvc

[olanti-p]

Summary

SUMMARY: Build "Fix false positive no return warnings in msvc"

Purpose of change

Fix warnings in lightmap.cpp:

src\lightmap.cpp(1148): warning C4715: '<lambda_5114cbcce979430cdc53a0f312cda1c1>::operator()': not all control paths return a value
src\lightmap.cpp(849): warning C4715: '<lambda_97faf4a5c59a05aa0c0ff99c9f164228>::operator()': not all control paths return a value

Describe the solution

Add a platform-independent unreachable() hint found at Stackoverflow (https://stackoverflow.com/a/65258501)

Describe alternatives you've considered

c++23

Testing

Game compiles with no warnings in VS, does not crash and burn when loading a save.