Use top-left grid cell when syncing staff charges

[StephenCWills]

This resolves #8500

[morfidon]

LGTM

This seems reasonable to me. InvGrid already treats negative entries as part of the same item in other places, so using std::abs(...) here looks like a consistency fix rather than a workaround.

[StephenCWills]

It's not about consistency or workaround. It needs to pass the index of the top-left grid cell to NetSendCmdChInvItem(). The top-left grid cell has a negative number in it for any item larger than 1x1 so we have to use std::abs() to find it. It's simply incorrect to do otherwise.

[StephenCWills]

Now that I think about it, this fix only applies to the source-side of the communications. A malicious client could still force an OOB on a remote client by passing the wrong index. This fix isn't quite sufficient.

[StephenCWills]

I added validation on the receiving end so this should be ready for review.