Don't create debug context if debug listener is not set

test/mjsunit/regress/regress-3960.js

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --allow-natives-syntax
 
-// Test that setting break point is works correctly when the debugger is
+// Test that setting break point works correctly when the debugger is
 // activated late, which leads to duplicate shared function infos.
 
 (function() {
   var Debug = %GetDebugContext().Debug;
 
   function listener(event, exec_state, event_data, data) {
     if (event != Debug.DebugEvent.Break) return;
     try {
       assertTrue(/foo/.test(exec_state.frame(0).sourceLineText()));
       break_count++;
     } catch (e) {
       exception = e;
     }
   }
 
   for (var i = 0; i < 3; i++) {
     var foo = function() { a = 1; }
     var exception = null;
     var break_count = 0;
     Debug.setListener(listener);
     if (i < 2) Debug.setBreakPoint(foo, 0, 0);
     assertTrue(/\[B\d\]a = 1/.test(Debug.showBreakPoints(foo)));
     foo();
     assertEquals(1, break_count);
     assertNull(exception);
   }
 
   Debug.setListener(null);
 })();