Fix undefined variable 'path' for http_login

This fixes an undefined variable for 'path' bug.

If no auth uri found, the find_auth_uri func should return a nil (at least that seems to be the original intention), so that run_host can bail like this:

	@uri = find_auth_uri
	if ! @uri
		print_error("#{target_url} No URI found that asks for HTTP authentication")
		return
	end

[See RM 8279]
http://dev.metasploit.com/redmine/issues/8279