fix autotargeting failure

the fallback to the original default was failing because
it was assuming rhost was already set, so it would always
go back to the first default target. now the auto_target? method
only returns true if can pull an auto_target_host

lib/msf/core/exploit/auto_target.rb

@@ -11,7 +11,7 @@ module Msf
     def auto_target?
       selected_target = targets[target_index]
       return false if selected_target.nil?
-      if selected_target.name =~ /Automatic/ && selected_target['AutoGenerated'] == true
+      if selected_target.name =~ /Automatic/ && selected_target['AutoGenerated'] == true  && auto_target_host
         true
       else
         false