macosx: fix crash when open track sync panel while movie is playing

Crash was introduced in 3e73ad2a8ab6d5e50ea80025d364edbe7ead1d1b (subsdelay filter) and
occurs when user or init methods set subsdelay to 0.
(cherry picked from commit fd9f59ec7cbda944c729118302c7434e5696896a)
Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

modules/gui/macosx/VideoEffects.m

@@ -409,6 +409,9 @@ static VLCVideoEffects *_o_sharedInstance = nil;
         else if( (NSInteger)strstr( psz_string, psz_name ) == NO )
             psz_string = (char *)[[NSString stringWithFormat: @"%s:%s", psz_string, psz_name] UTF8String];
     } else {
+        if( !psz_string )
+            return;
+
         psz_parser = strstr( psz_string, psz_name );
         if( psz_parser )
         {