Fix undefined method error

[FixRM rapid7#8334]
[FixRM rapid7#8333]

Author: wchen-r7

modules/auxiliary/scanner/http/nginx_source_disclosure.rb

@@ -72,7 +72,10 @@ def run_host(ip)
 					'uri'     => "#{uri}#{get_source}",
 				}, 25)
 
-			if res
+			if res.nil?
+				print_error("Connection timed out")
+				return
+			else
 				version = res.headers['Server']
 				http_fingerprint({ :response => res })
 			end