readd fix for rapid7#1219

Author: jvazquez-r7

modules/exploits/windows/browser/ie_cdwnbindinfo_uaf.rb

@@ -90,6 +90,9 @@ def get_target(agent)
 			os_name = 'Windows Vista'
 		when '6.1'
 			os_name = 'Windows 7'
+		else
+			# OS not supported
+			return nil
 		end
 
 		targets.each do |t|