fix corner case in collapse_vars (#5274)

fixes #5273

lib/compress.js

@@ -1962,21 +1962,27 @@ Compressor.prototype.compress = function(node) {
                     can_replace = false;
                     node.right.transform(scanner);
                     clear_write_only(candidate);
-                    var assign = make_node(AST_Assign, node, {
+                    var folded;
+                    if (abort) {
+                        folded = candidate;
+                    } else {
+                        abort = true;
+                        lhs.definition().fixed = false;
+                        folded = make_node(AST_Binary, candidate, {
+                            operator: compound,
+                            left: lhs,
+                            right: rvalue,
+                        });
+                    }
+                    return make_node(AST_Assign, node, {
                         operator: "=",
                         left: node.left,
                         right: make_node(AST_Binary, node, {
                             operator: node.operator.slice(0, -1),
-                            left: abort ? candidate : make_node(AST_Binary, candidate, {
-                                operator: compound,
-                                left: lhs,
-                                right: rvalue,
-                            }),
+                            left: folded,
                             right: node.right,
                         }),
                     });
-                    abort = true;
-                    return assign;
                 }
                 // Stop immediately if these node types are encountered
                 if (should_stop(node, parent)) {