fix collapse_vars regression in destructuring (#2897)

fixes #2896

lib/compress.js

@@ -1512,6 +1512,7 @@ merge(Compressor.prototype, {
             }
 
             function may_modify(sym) {
+                if (!sym.definition) return true; // AST_Destructuring
                 var def = sym.definition();
                 if (def.orig.length == 1 && def.orig[0] instanceof AST_SymbolDefun) return false;
                 if (def.scope.get_defun_scope() !== scope) return true;