Failsafe requirejs unset/reset

Introduced in 5c75aa8 it might happen,
that there is no `define` or `require` defined. handle this gracefully.

CHANGES.rst

@@ -4,6 +4,9 @@ Changelog
 1.10 (unreleased)
 -----------------
 
+- failsafe Unset and reset ``define`` and ``require``
+  [petschki]
+
 - PEP 8.
   [thet]
 

plone/app/widgets/static/requirejs-reset.js

@@ -1,2 +1,2 @@
-var define = _old_define;
-var require = _old_require;
+if(typeof _old_define !== 'undefined') { var define = _old_define; }
+if(typeof _old_require !== 'undefined') { var require = _old_require; }

plone/app/widgets/static/requirejs-unset.js

@@ -1,4 +1,4 @@
-var _old_define = define;
-var _old_require = require;
+if(typeof define !== 'undefined') { var _old_define = define; }
+if(typeof require !== 'undefined') { var _old_require = require; }
 define = undefined;
 require = undefined;