fix corner case in collapse_vars (#4002)

fixes #4001

test/compress/ie8.js

@@ -2559,3 +2559,37 @@ issue_3999: {
         "1",
     ]
 }
+
+issue_4001: {
+    options = {
+        collapse_vars: true,
+        ie8: true,
+        inline: true,
+        reduce_vars: true,
+        sequences: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        console.log(function(a) {
+            function f() {
+                return a;
+                var b;
+            }
+            var c = f();
+            (function g() {
+                c[42];
+                f;
+            })();
+            (function a() {});
+        }(42));
+    }
+    expect: {
+        function f() {
+            return a;
+        }
+        var a;
+        console.log((a = 42, void f()[42], void function a() {}));
+    }
+    expect_stdout: "undefined"
+}