Rescue an IOError on channel double-close.

This was causing output from python meterpreter
commands run on OSX to be discarded when the error
was raised, making cmd_exec not-so-useful.

lib/msf/core/post/common.rb

@@ -111,7 +111,13 @@ module Msf::Post::Common
         o << d
       end
       o.chomp! if o
-      process.channel.close
+
+      begin
+        process.channel.close
+      rescue IOError => e
+        # Channel was already closed, but we got the cmd output, so let's soldier on.
+      end
+
       process.close
     when /shell/
       o = session.shell_command_token("#{cmd} #{args}", time_out)