[zeditor] Crash-to-desktop when right-clicking Link To... in FFC Editor

Steps to recreate:

1. Screen->Freeform Combos
2. Click Edit in "Choose Freeform Combo" window
3. rclick the "Link to:" dropdown in the Data tab of the FFC Editor

Here in drop_down_list.cpp is the assertion that failed because event == 8 (geRCLICK)

Code:

---

int32_t DropDownList::onEvent(int32_t event, MessageDispatcher& sendMessage)
{
        assert(event == geCHANGE_SELECTION);
        if(onSelectFunc)
                onSelectFunc(listData->getValue(alDialog->d1));
        if(message >= 0)
                sendMessage(message, listData->getValue(alDialog->d1));
        return -1;
}

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Cannot recreate in a Release build... this crash appears to only affect developers using Debug builds.