Fix crash when we change systray setting from folder when we close kmail

We close kmail, it deletes foldercollection which calls updatesystemtray
which is in state deleted
=> crash

kmkernel.cpp

@@ -1446,7 +1446,7 @@ bool KMKernel::haveSystemTrayApplet() const
 
 void KMKernel::updateSystemTray()
 {
-  if ( mSystemTray ) {
+  if ( mSystemTray && !the_shuttingDown ) {
     mSystemTray->updateSystemTray();
   }
 }