cron(8): use proper variable to determine mailer process status

While the mailer is normally opened/set if the mailto is set, this is not
the case if the grandchild actually didn't produce any output. This change
corrects the situation to only attempt to kill/close the mail process if it
was actually opened in the first place.

The reporter initially stumbled on the -n (suppress mail on success) flag
leading to a SIGKILL of the process group, but simultaneously
discovered/reported the behavior with !-n jobs if MAILTO was set and no
output happened.

All of these places that are checking mailto should actually be checking
whether mail is set, so do that for consistency+correctness.

This set of bugs were introduced by r352668.

Submitted by: sigsys@gmail.com
Reported by: sigsys@gmail.com