GUI: mux: ensure output name uniqueness before adding to queue

Otherwise the following steps would end up overwriting an existing
file:

* add a file
* start multiplexing
* re-use the same settings
* don't change anything regarding files & track selection (but modify
  e.g. a track language)
* start multiplexing again

Fixes #2052.

NEWS.md

@@ -53,6 +53,9 @@
   again. Fixes #2045.
 * mkvmerge: when appending fails the error message details (e.g. "the number
   of channels differs: 1 and 2") were often not output. Fixes #2046.
+* MKVToolNix GUI: multiplexer: if the GUI is set to ensure unique output file
+  names, it will now verify that right before starting to multiplex/adding the
+  job to the queue, too. Fixes #2052.
 
 ## Build system changes
 

src/mkvtoolnix-gui/merge/tab.cpp

@@ -433,6 +433,7 @@ void
 Tab::addToJobQueue(bool startNow,
                    boost::optional<Util::Settings::ClearMergeSettingsAction> clearSettings) {
   updateConfigFromControlValues();
+  setOutputFileNameMaybe();
 
   if (!isReadyForMerging() || !checkIfOverwritingIsOK())
     return;