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

Coming out of #10919.

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

https://github.com/keymanapp/keyman/blob/763bc967399e0e0226c610c34fe34c50ffaed113/core/src/kmx/kmx_processevent.cpp#L107-L111

The debug list is initialized immediately above:

https://github.com/keymanapp/keyman/blob/763bc967399e0e0226c610c34fe34c50ffaed113/core/src/kmx/kmx_processevent.cpp#L92-L98

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.