Fix consumption of mixed ammo in magazines

[olanti-p]

Summary

SUMMARY: Bugfixes "Fix consumption of mixed ammo in magazines"

Purpose of change

When reloading a magazine with a mix of favorite and non-favorite ammo the 2 types stay as separate stacks inside the magazine.
But the gun code isn't "quite there" when it comes to handling mixed ammo, so the part where they're consumed mistakenly uses back() instead of front() to refer to the stack to remove, which ends up with wrong stack of ammo being erased.

Example repro steps:

1. Debug spawn Glock 31
2. Unload the glock
3. Drop 14 ammo
4. Mark the remaining as favorite
5. Pick up dropped ammo
6. Load glock with 1 piece of favorite ammo
7. Load glock with 14 pieces of non-favorite ammo
8. Shoot once
9. See 14 ammo being consumed

Describe the solution

Fix back() -> front()

Testing

Repeated repro steps, bug is fixed