fix(windows): incxstr could run over buffer with malformed data

Fixes #4591.

I fixed incxstr in 4 places:

1. Common/Core: kmx_xstring.cpp
2. Engine: xstring.cpp
3. Test project importkeyboard importkeyboard.cpp
4. Test project m-to-p m-to-p.cpp

I updated mcompile to remove its own copy of incxstr (identical to that
in xstring.cpp) to reduce WETness but opted not to do so for the test
apps, which are pretty much throwaway anyway.

I note that there is more work we could do here; we need to check every
character as we increment so we don't miss a `U+0000` end of string with
malformed data. But I would like to tackle that as a separate job at
some point in the future after Core integration.