fix(developer): prevent buffer overrun in `u16tok`

[mcdurdin]

Relates-to: #11814 See-also: #11894

@keymanapp-test-bot skip

[keymanapp-test-bot[bot]]

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[mcdurdin]

I've added a few more tests to #11894 which builds on this.

[markcsinclair]

I merged #11910 into #11894, so I'm waiting for #11910 to be merged into master before I submit #11894 for review. Josh has approved #11910, so I'm hoping you will merge it into master soon.

[keyman-server]

Changes in this pull request will be available for download in Keyman version 18.0.73-alpha