Fixes crash during save due to invalid ammo_location

[satheon49]

Summary

SUMMARY: Bugfixes "Fixes crash due to invalid ammo_location"

Purpose of change

Fixes #50399
The problem was that due to trying to reload a bow a container with appropriate ammunition was set as an ammo_location for the bow. However if that container was later dropped the game would crash during save due to the ammo_location no longer being valid.

Describe the solution

ammo_location is only saved if it is valid.

Describe alternatives you've considered

Preventing longbows from ever being reloadable. However this would not fix existing saves that already have an ammo_location set (and worse outside from manually removing the ammo_location from the save there would no longer be a way to fix it by "reloading" the bow and setting a new ammo_location.

In addition this problem might not be restricted to bows. If ANY ammo_location becomes invalid for whatever reason and the player does not reload between that moment and the next save the game would crash.

Testing

Load the save referenced in #50399 and dropped the double rifle case and saved. The game no longer crashed.
For debug testing:

1. Spawn bow, 2 arrows and quiver
2. Wear quiver, wield bow
3. Pick up 1 arrow into the quiver
4. "Reload bow"-> pick quiver as source
5. Drop quiver
6. Save

[mqrause]

This sounds like it possibly also fixes #50029.

[satheon49]

Very likely. The crash for 50399 was caused by a call of item_location::calc_index too (with item_location being the ammo_location which was null at that point)

[Qrox]

Does this fix #39930?

[satheon49]

I could not reproduce a crash with smashing the vehicle (with or without PR) but saving with an unloaded vehicle is fixed by this PR too.