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

Crash was introduced in 3e73ad2a (subsdelay filter) and
occurs when user or init methods set subsdelay to 0.
(cherry picked from commit fd9f59ec)
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 )
         {