Do not update the mask if we do not have a surface

mMask serves as a cache to remember what we've sent, the source of truth
for the value is window()->mask().
No need to store values that we are going to discard, because it will
confuse the state of newly created windows.

Change-Id: I6aa3da82c7f09c7ef90d0f7060f292fb042730f0
Pick-to: 5.15 6.2
Reviewed-by: David Edmundson <davidedmundson@kde.org>

src/plugins/platforms/wayland/qwaylandwindow.cpp

@@ -478,15 +478,15 @@ void QWaylandWindow::lower()
 
 void QWaylandWindow::setMask(const QRegion &mask)
 {
+    QReadLocker locker(&mSurfaceLock);
+    if (!mSurface)
+        return;
+
     if (mMask == mask)
         return;
 
     mMask = mask;
 
-    QReadLocker locker(&mSurfaceLock);
-    if (!mSurface)
-        return;
-
     if (mMask.isEmpty()) {
         mSurface->set_input_region(nullptr);