New system for silent require errors. Use the new function

stdnse.silent_require. The Lua require function is back in its usual spot
(_G.require).

scripts/mysql-variables.nse

@@ -38,7 +38,7 @@ categories = {"discovery", "intrusive"}
 require 'shortport'
 require 'stdnse'
 require 'mysql'
-require 'openssl'
+stdnse.silent_require 'openssl'
 
 dependencies = {"mysql-brute", "mysql-empty-password"}