utf8_to_uvchr_buf() make behavior match docs

utf8_to_uvchr_buf() make behavior match docs

For well formed input, there is no change.  But for malformed it wasn't
returning the documented length when warnings were enabled, and not
always the documented value when they were disabled.

This is implemented as an inline function, called from both the macro
and the Perl_ form.

Devel::PPPort has sufficient tests for this.