macosx: fixed crash due to race condition on launch

The main window may be asked to update its content prior to VLCFSPanel being fully initialized
(cherry picked from commit 8ef447a49d49598e5b022c4b2f03a3b8a6a24d98)

modules/gui/macosx/MainWindow.m

@@ -1470,6 +1470,7 @@ static VLCMainWindow *_o_sharedInstance = nil;
 
     [o_time_sld setEnabled: b_seekable];
     [self updateTimeSlider];
+    if ([o_fspanel respondsToSelector:@selector(setSeekable:)])
         [o_fspanel setSeekable: b_seekable];
 
     PL_LOCK;