avcodec/videotoolboxenc: don't wait when flushing data

because there is run in thread mode, few times will block
the workflow at the wait, so check the status is flushing data,
don't wait when flushing data.

Signed-off-by: Tian Qi <tianqi@kuaishou.com>
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>
Signed-off-by: Rick Kern <kernrj@gmail.com>

libavcodec/videotoolboxenc.c

@@ -294,7 +294,7 @@ static int vtenc_q_pop(VTEncContext *vtctx, bool wait, CMSampleBufferRef *buf, E
         return 0;
     }
 
-    while (!vtctx->q_head && !vtctx->async_error && wait) {
+    while (!vtctx->q_head && !vtctx->async_error && wait && !vtctx->flushing) {
         pthread_cond_wait(&vtctx->cv_sample_sent, &vtctx->lock);
     }
 
@@ -310,6 +310,7 @@ static int vtenc_q_pop(VTEncContext *vtctx, bool wait, CMSampleBufferRef *buf, E
         vtctx->q_tail = NULL;
     }
 
+    vtctx->frame_ct_out++;
     pthread_mutex_unlock(&vtctx->lock);
 
     *buf = info->cm_buffer;
@@ -321,7 +322,6 @@ static int vtenc_q_pop(VTEncContext *vtctx, bool wait, CMSampleBufferRef *buf, E
     }
     av_free(info);
 
-    vtctx->frame_ct_out++;
 
     return 0;
 }