fix insert item segfault

[marilynias]

Summary

Bugfixes "fix segfault when trying to insert items into container not carried"

Purpose of change

In #79518 I forgot to check if there even is a carrier in the first place, leading to a segfault.
fix #79548

Describe the solution

carrier != nullptr before carrier->is_avatar()

Describe alternatives you've considered

Make a default implementation for similar to item_location::nowhere, character::noone, which would touch alot more code since the it seems to be standard to check if Character == nullptr

Testing

can insert item from issue.