fix crash in mix transition with consumer producer

See https://forum.shotcut.org/t/adding-to-timeline-a-mlt-as-a-clip-with-
different-fps-crashes-shotcut/46418

src/modules/avformat/filter_swresample.c

@@ -64,7 +64,8 @@ static int filter_get_audio(mlt_frame frame,
     }
 
     if (in.samples == 0) {
-        // Noting to convert.
+        // Nothing to convert.
+        *samples = 0;
         return error;
     }