BUG/MINOR: cfgparse-listen: fix the argument checked by "email-alert"

BUG/MINOR: cfgparse-listen: fix the argument checked by "email-alert"

The "from", "mailers", "myhostname" and "to" options of "email-alert"
each check that a value was passed, but they test args[1], which is the
option's own name instead fo args[2]. As a consequence an empty args[2]
was accepted. The same copy-paste mistake made the "level" option error
handling report the option name instead of the wrong value.

This may be backported to all supported versions.