fix crash in unsafe replacement of undefined

remove extraneous call to AST_SymbolRef.reference()

closes #1443

test/compress/issue-1443.js

@@ -0,0 +1,69 @@
+// tests assume that variable `undefined` not redefined and has `void 0` as value
+
+unsafe_undefined: {
+    options = {
+        if_return: true,
+        unsafe: true
+    }
+    mangle = {}
+    input: {
+        function f(undefined) {
+            return function() {
+                if (a)
+                    return b;
+                if (c)
+                    return d;
+            };
+        }
+    }
+    expect: {
+        function f(n) {
+            return function() {
+                if (a)
+                    return b;
+                if (c)
+                    return d;
+                else
+                    return n;
+            };
+        }
+    }
+}
+
+keep_fnames: {
+    options = {
+        if_return: true,
+        unsafe: true
+    }
+    mangle = {
+        keep_fnames: true
+    }
+    input: {
+        function f(undefined) {
+            return function() {
+                function n(a) {
+                    return a * a;
+                }
+                if (a)
+                    return b;
+                if (c)
+                    return d;
+            };
+        }
+    }
+    expect: {
+        function f(r) {
+            return function() {
+                function n(n) {
+                    return n * n;
+                }
+                if (a)
+                    return b;
+                if (c)
+                    return d;
+                else
+                    return r;
+            };
+        }
+    }
+}