Fix ASAN warnings in XDGForeignV2Interface

Summary:
The current code captures a char* from a wayland event in a lambda.
By creating the QString first when we capture that by value we'll
implicitly shallow copy it.

The other issue was
someHash.erase(it);
*it->foo();

Even though where the iterator points to is still valid, the iterator
itself is an object that gets modified in erase which ASAN didn't like.

Test Plan:
Ran test.
Those particular errors went away
Tests passed

Subscribers: plasma-devel, #frameworks

Tags: #plasma_on_wayland, #frameworks

Differential Revision: https://phabricator.kde.org/D9319