Check the return value from utf8_to_ucs2 instead of whether or not uv
is NULL. That's more correct and doesn't depend on the error behavior
of utf8_to_ucs2. In practice, we'll never see this though since we
pass utf8_to_ucs2 a well formed string.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D13918