fix monkey NPE

  Bug: 6445628

Change-Id: I4a4e2aa2e76596501deea5b6b6b05ea5bcda396e
diff --git a/src/com/android/browser/PieControl.java b/src/com/android/browser/PieControl.java
index abe8d37..7c4291c 100644
--- a/src/com/android/browser/PieControl.java
+++ b/src/com/android/browser/PieControl.java
@@ -115,7 +115,9 @@
         int n = mUiController.getTabControl().getTabCount();
         mTabsCount.setText(Integer.toString(n));
         Tab tab = mUiController.getCurrentTab();
-        mForward.setEnabled(tab.canGoForward());
+        if (tab != null) {
+            mForward.setEnabled(tab.canGoForward());
+        }
         return true;
     }