fix pre-existing issue

- chained assignment

lib/compress.js

@@ -297,7 +297,9 @@ merge(Compressor.prototype, {
                     && node.operator == "="
                     && node.left instanceof AST_SymbolRef) {
                     var d = node.left.definition();
-                    if (HOP(safe_ids, d.id) && safe_to_assign(d, node.right)) {
+                    if (HOP(safe_ids, d.id)
+                        && safe_to_read(d)
+                        && safe_to_assign(d, node.right)) {
                         d.references.push(node.left);
                         d.fixed = function() {
                             return node.right;

test/compress/reduce_vars.js

@@ -2110,6 +2110,27 @@ var_assign_5: {
     expect_stdout: "2 undefined"
 }
 
+var_assign_6: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        !function() {
+            var a = function(){}(a = 1);
+            console.log(a);
+        }();
+    }
+    expect: {
+        !function() {
+            var a = function(){}(a = 1);
+            console.log(a);
+        }();
+    }
+    expect_stdout: "undefined"
+}
+
 immutable: {
     options = {
         evaluate: true,