QIcon::setIsMask: don't dereference nullptr

QIcon::detach might result in the d pointer being nullptr, and other
code tests for that condition and allocates the d pointer explicitly.

Change the order of the tests in setIsMask as well, fix static analzyer
warning 0a4723ca2d216ec8883a3d412aeded4c.

Pick-to: 6.1
Change-Id: Idca12f230036362f13cbaee1ae4f0e5ce6726015
Reviewed-by: David Skoland <david.skoland@qt.io>
Reviewed-by: Oliver Eftevaag <oliver.eftevaag@qt.io>
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/gui/image/qicon.cpp

@@ -1377,10 +1377,10 @@ bool QIcon::hasThemeIcon(const QString &name)
 */
 void QIcon::setIsMask(bool isMask)
 {
+    if (d)
+        detach();
     if (!d)
         d = new QIconPrivate(new QPixmapIconEngine);
-    else
-        detach();
     d->is_mask = isMask;
 }