Don't clear active override after processing incoming Triforce piece

[fenhl]

The code for handling an incoming item from the network distinguishes 3 cases: refills (since #2069), Triforce pieces, and everything else. For Triforce pieces, the code was calling clear_override even though none of the code leading to it calls activate_override — incoming items use the incoming override queue, not the active override. This could cause an item being picked up at the same time to lose its override and act as the “base item” used for it in the item table, usually either a Gerudo mask or a blue rupee.

If I'm right, this fixes #257.

Testing

~~I intend to test this using a BizHawk external tool that runs a short TAS to open a chest and injects an incoming item during the right frame for the bug to trigger.~~ See following comments.