Add checks for items, skills and bionics of professions

Addictions are already checked when creating an instance of addiction in profession::load_profession
recipes: check for book-to-learn-from exists and is a book else print a debug message
handle re-definitions of vehicle parts gracefully
handle unknown part types in vehicle prototypes
check spawn definitions in vehicle protoypes

Removed the GUITAR spawn from fire trucks.

The item currently does not exists, can be added later when #1980 gets fixed.
Make map::spawn_item ignore item ids that would lead to a null item on the map.