utf8: modernize utf16 inline calls a bit

utf8: modernize utf16 inline calls a bit

Let's fix an indentation issue.

Let's avoid yoda comparisons.

Let's drop unnecessary ().

Let's make sure we convert 16bit values to 32bit before shifting them by
10bit to the left, to avoid overflows.

Let's avoid comparisons between signed literals and unsigned variables,
in particular if the literals are outside of the minimum range C
requires for "int".