Fix #3886 - Backtrace for #check when session is invalid

If the user supplies an invalid session (as in not on the session
list), it will cause a backtrace, because the setup method from
Msf::PostMixin isn't actually called.

We have thought about implementing this in a new OptSession instead.
But you can't use or even pass framework to option_container.rb, so
this is NOT possible.

The original PR was #3956.
Land #4300, @wchen-r7's fix for #3886