check to see if the real title bar has no parent

This works around a monkey-found crash that is unlikely to
be triggerable by a user.

Fixes http://b/issue?id=2150746
diff --git a/src/com/android/browser/BrowserActivity.java b/src/com/android/browser/BrowserActivity.java
index babb43c..589fff0 100644
--- a/src/com/android/browser/BrowserActivity.java
+++ b/src/com/android/browser/BrowserActivity.java
@@ -1071,6 +1071,9 @@
      * context menu.
      */
     /* package */ void showTitleBarContextMenu() {
+        if (null == mTitleBar.getParent()) {
+            return;
+        }
         openContextMenu(mTitleBar);
     }