Fix #4156, cmdstager NoMethodError undefined method `stop'

[wchen-r7]

This fixes: #4156

To verify the bug, you can do:

• ruby -run -e httpd . -p 8181 and use this as your fake server
• Start msfconsole
• use exploit/multi/http/struts_code_exec
• set rhost [target]
• set lhost [ip]
• set rport 8181
• set PAYLOAD windows/meterpreter/reverse_https
• set URI /
• exploit
• You should see "NoMethodError undefined method `stop' for nil:NilClass"

And then:

• Apply the patch, repeat the same steps. You should no longer see the error.

[wvu]

@wchen-r7: I changed use use exploit/multi/http/struts_code_exec to use exploit/multi/http/struts_code_exec and added set URI /. Also, I used rvmsudo, since the TFTP thingy needs root to bind.