bug(core): debug lists may not be properly terminated for failure paths in kmx processor

[mcdurdin]

Coming out of #10919.

For example, the debug list is not terminated in this situation:

keyman/core/src/kmx/kmx_processevent.cpp

Lines 107 to 111 in 763bc96

	if (kbd->StartGroup[BEGIN_UNICODE] == (KMX_DWORD) -1) {
	DebugLog("Non-Unicode keyboards are not supported.");
	m_core_state = nullptr;
	return FALSE;
	}

The debug list is initialized immediately above:

keyman/core/src/kmx/kmx_processevent.cpp

Lines 92 to 98 in 763bc96

	state->debug_items().clear();
	if(state->debug_items().is_enabled()) {
	CreateInternalDebugItems();
	} else {
	// We want to have a clean debug state even if it is not in use
	state->debug_items().push_end(m_actions.Length(), 0);
	}

In most cases, a failure path would not have the consumer reading the debug list, but a separate bug documented in #10915 shows how this is still not right at present, and it does lead to crashes in those scenarios due to running off the end of the list.