add_file(): ensure track_will_expand is defined

That's a potential bug, but conditions for it to happen are very rare.

Method is only called from https://github.com/metabrainz/picard/blob/9eda85bf3a18612d05204894dea7d9d4166ae69b/picard/file.py#L653
 and it only runs if the parent actually changed

picard/track.py

@@ -155,6 +155,7 @@ def linked_files(self):
         return self.files
 
     def add_file(self, file, new_album=True):
+        track_will_expand = False
         if file not in self.files:
             track_will_expand = self.num_linked_files == 1
             if not self.files:  # The track uses original metadata from the file only