fix corner case in unsafe_proto

lib/ast.js

@@ -310,9 +310,9 @@ var AST_Scope = DEFNODE("Scope", "variables functions uses_with uses_eval parent
     },
     clone: function(deep) {
         var node = this._clone(deep);
-        node.variables = this.variables.clone();
-        node.functions = this.functions.clone();
-        node.enclosed = this.enclosed.slice();
+        if (this.variables) node.variables = this.variables.clone();
+        if (this.functions) node.functions = this.functions.clone();
+        if (this.enclosed) node.enclosed = this.enclosed.slice();
         return node;
     }
 }, AST_Block);

test/compress/reduce_vars.js

@@ -5303,7 +5303,7 @@ issue_2774: {
     expect_stdout: "undefined"
 }
 
-issue_2799: {
+issue_2799_1: {
     options = {
         reduce_funcs: true,
         reduce_vars: true,
@@ -5336,3 +5336,27 @@ issue_2799: {
     }
     expect_stdout: "15"
 }
+
+issue_2799_2: {
+    options = {
+        reduce_vars: true,
+        unsafe_proto: true,
+        unused: true,
+    }
+    input: {
+        (function() {
+            function foo() {
+                Function.prototype.call.apply(console.log, [ null, "PASS" ]);
+            }
+            foo();
+        })();
+    }
+    expect: {
+        (function() {
+            (function() {
+                (function() {}).call.apply(console.log, [ null, "PASS" ]);
+            })();
+        })();
+    }
+    expect_stdout: "PASS"
+}