qt follow stream: set the file name before opening

If we don't, Save As will always fail.

(I broke this in 6a992182ce)

Change-Id: I4abfe495af3aee470331ae5464acbd7d08028bf0
Reviewed-on: https://code.wireshark.org/review/15547
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>

ui/qt/follow_stream_dialog.cpp

@@ -268,11 +268,11 @@ void FollowStreamDialog::saveAs()
     if (!file_name.isEmpty()) {
         QTextStream out(&file_);
 
+        file_.setFileName(file_name);
         if (!file_.open(QIODevice::WriteOnly)) {
             open_failure_alert_box(file_name.toUtf8().constData(), errno, TRUE);
             return;
         }
-        file_.setFileName(file_name);
 
         save_as_ = true;