QFileDialog: Check whether sizeHint() is valid before resizing.

Fixes a regression appearing in 5.5 where QtWidgets-based file dialogs
showed up too small on Windows.

Change-Id: I5f74ab01d4368d67ab79d3a1b145fb63bdd69ca0
Reviewed-by: Shawn Rutledge <shawn.rutledge@theqtcompany.com>

src/widgets/dialogs/qfiledialog.cpp

@@ -2822,7 +2822,9 @@ void QFileDialogPrivate::init(const QUrl &directory, const QString &nameFilter,
     qFileDialogUi->sidebar->hide();
 #endif
 
-    q->resize(q->sizeHint());
+    const QSize sizeHint = q->sizeHint();
+    if (sizeHint.isValid())
+       q->resize(sizeHint);
 }
 
 /*!