Fix wmap_sites -a exception on missing url

plugins/wmap.rb

@@ -127,12 +127,15 @@ class Plugin::Wmap < Msf::Plugin
       while (arg = args.shift)
         case arg
         when '-a'
-          s = add_web_site(args.shift)
+          site = args.shift
+          if site
+            s = add_web_site(site)
             if s
               print_status("Site created.")
             else
               print_error("Unable to create site")
             end
+          end
         when '-d'
           del_idx = args
           if del_idx