Fix trade crash in certain situations

[dseguin]

Summary

None

Purpose of change

Fixes #52014

Describe the solution

Store the item_location itself instead of storing a copy of the pointer.

Describe alternatives you've considered

There's probably some pointer magic that could sort this out as well, but this seems like the safer option.

Testing

1. Load save file from the issue
2. Attempt to trade using items from various locations (vehicle cargo, on person, inside container, etc.)
3. No more seg faults

Additional context

[ferociousdork]

If you wish, you can change:

Cataclysm-DDA/src/npctrade.cpp

Line 86 in 81ae267

for( item_pricing ip : sorted_stuff ) {

to capture by reference as well. That copy is the reason it crashed in the first place.

[dseguin]

Cheers, I would have never thought of that. I've updated the solution.