fix catch variable reference in IE8 (#1587)

`AST_Scope.def_variable()` will overwrite `AST_Symbol.thedef`, so save a copy before calling.

fixes #1586

lib/scope.js

@@ -212,9 +212,10 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options){
         self.walk(new TreeWalker(function(node, descend) {
             if (node instanceof AST_SymbolCatch) {
                 var name = node.name;
+                var refs = node.thedef.references;
                 var scope = node.thedef.scope.parent_scope;
                 var def = scope.find_variable(name) || self.globals.get(name) || scope.def_variable(node);
-                node.thedef.references.forEach(function(ref) {
+                refs.forEach(function(ref) {
                     ref.thedef = def;
                     ref.reference(options);
                 });