Don't try to get non-existent spellbook spells

[RenechCDDA]

Summary

None

Purpose of change

• Fixes Using the structural imager makes you learn an invalid spell that causes a segfault when cast. #73749

Describe the solution

Don't bother accessing the player's spellbook or knowledge - just get the corresponding spell itself and return the translated name.

Add an additional bit of safety to known_magic::get_spell. It was returning a spell reference even if the reference was known to be bad. Return some dummy data instead. Trying to access the spellbook(std::map) that doesn't contain that requested key inserts that key into the map.

Describe alternatives you've considered

Testing

Additional context

[Sofistico]

Thx for the catch! I was the one who did the name in the casting waity thingy.