sort, who: prefer free+malloc to realloc when contents are irrelevant

sort, who: prefer free+malloc to realloc when contents are irrelevant

This change was prompted by the previous one: I audited the code
looking for similar examples.  Too bad valgrind doesn't catch this.
* src/sort.c (check, mergefps): xrealloc -> free + xmalloc
* src/who.c (print_user): Likewise.