KMail: fix queryClose() always returning true

KConfigGui::sessionConfig() always returns a valid pointer, so the condition
would always be true, which caused KMail to terminate whenever the main window
was closed (even when systray was enabled).

qApp->isSavingSession() is the right way to tell whether session saving is
in progress (even if we handle it by our own config).

kmmainwin.cpp

@@ -208,7 +208,7 @@ void KMMainWin::saveProperties(KConfigGroup &config)
 
 bool KMMainWin::queryClose()
 {
-    if (kmkernel->shuttingDown() || KConfigGui::sessionConfig() || mReallyClose) {
+    if (kmkernel->shuttingDown() || qApp->isSavingSession() || mReallyClose) {
         return true;
     }
     return kmkernel->canQueryClose();