Prevent switching to NULL or own window in the "vt_proc_window_switch"
function. This fixes an issue where X11 keyboard input can appear
stuck. The cause of the problem is a duplicate TTY device window
switch IOCTL during boot, which leaves the "vt_switch_timer" running,
because the current window is already selected. While at it factor out
some NULL checks.

PR: 200032
Differential Revision: https://reviews.freebsd.org/D2480
Reported by: several people
MFC after: 1 week
Reviewed by: emaste