who: avoid new warning from upcoming gcc-7

who: avoid new warning from upcoming gcc-7

* src/who.c (idle_string): This function would fail to compile
with -Werror and today's built-from-git gcc due to this warning:
src/who.c: In function 'print_user':
src/who.c:201:36: error: may write format character ':' at offset 4 \
  past the end of the destination [-Werror=format-length=]
           sprintf (idle_hhmm, "%02d:%02d",
                                    ^~~~~
The fix is to use an assertion to inform gcc of the existing
invariant that guarantees the number of hours is less than 24.