[CPP] Fix map crash when counting trusts

[ampitere]

I affirm:

• I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• I have read and understood the Contributing Guide and the Code of Conduct.
• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

Fixes a map crash due to party leader being null when they are initially zoning. Resolves #5844

Steps to test these changes

1. Make a group with 2 or more
2. Be in the same zone together and have the party leader zone out and in

[zach2good]

No need to make a copy of the trusts vector here, same for below.

trustCount is init'd as zero:

size_t trustCount = 0;

The size of a vector is zero when it's empty.

So you can simplify this block (and below) with something like:

trustCount = static_cast<CCharEntity*>(m_PLeader)->PTrusts.size();

[ampitere]

Simplified both this line and 867.