w -sf and w -si segfault when systemd support is enabled

To reproduce: configure with systemd support, and run w -sf or w -si.

My attempt at diagnosing the issue:

• On line 831, showinfo is called with utmp = NULL
• On line 622, print_from is called with session = NULL
• With neither utmp or session, print_from dereferences a null pointer

I'm not too familiar with the code base, but a conditional check to pass the session like on line 581 might fix the issue.