vf_thumbnail: forward request_frame at least once.

The current version relied on poll_frame to request
the very first frame, that would not work if the
surrounding code does not call poll_frame.

libavfilter/vf_thumbnail.c

@@ -176,11 +176,11 @@ static int request_frame(AVFilterLink *link)
 
     /* loop until a frame thumbnail is available (when a frame is queued,
      * thumb->n is reset to zero) */
-    while (thumb->n) {
+    do {
         int ret = avfilter_request_frame(link->src->inputs[0]);
         if (ret < 0)
             return ret;
-    }
+    } while (thumb->n);
     return 0;
 }