sd: fix STL assertion "dereference past-the-end iterator"

(regression from 967a386b)

Change-Id: I9408170eb155f433922e5b3887059078a141d78a

sd/source/ui/view/ViewShellManager.cxx

@@ -767,7 +767,8 @@ void ViewShellManager::Implementation::UpdateShellStack (void)
     CreateShells();
 
     // Update the pointer to the top-most active view shell.
-    mpTopViewShell = maActiveViewShells.begin()->mpShell;
+    mpTopViewShell = (maActiveViewShells.empty())
+        ? 0 : maActiveViewShells.begin()->mpShell;
 
 
     // 2. Create the internal target stack.