Fix undefined method error

[FixRM rapid7#8339]

Author: wchen-r7

modules/auxiliary/scanner/http/dolibarr_login.rb

@@ -92,6 +92,11 @@ def do_login(user, pass)
 			return :abort
 		end
 
+		if res.nil?
+			print_error("Connection timed out")
+			return :abort
+		end
+
 		location = res.headers['Location']
 		if res and res.headers and (location = res.headers['Location']) and location =~ /admin\//
 			print_good("#{@peer} - Successful login: \"#{user}:#{pass}\"")