__setrunelocale: Fix asprintf(3) failure not returning an error.

Also fix the style of the asprintf(3) call in __collate_load_tables_l().
Both of these lines were modified away from snprintf(3) during the
import from DragonFly/Illumos.

Reviewed by: jilles (briefly over shoulder)
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon