Fix rapid7#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 rapid7#3956.

Author: wchen-r7

lib/msf/base/simple/auxiliary.rb

@@ -113,6 +113,8 @@ def self.check_simple(mod, opts)
     # be normalized
     mod.validate
 
+    mod.setup
+
     # Run check
     mod.check
   end

lib/msf/base/simple/exploit.rb

@@ -182,6 +182,8 @@ def self.check_simple(mod, opts)
     # be normalized
     mod.validate
 
+    mod.setup
+
     # Run check
     mod.check
   end

lib/msf/core/post/common.rb

@@ -3,6 +3,8 @@
 module Msf::Post::Common
 
   def rhost
+    return nil unless session
+
     case session.type
     when 'meterpreter'
       session.sock.peerhost