vda: error out if decoded CVPixelBuffer is empty

On some video samples, VDA silently fails to decode frames and returns
kVDADecoderNoErr. Error out in these cases to avoid producing AVFrames with
empty planes.
Signed-off-by: Stefano Pigozzi <stefano.pigozzi@gmail.com>

libavcodec/vda_h264.c

@@ -345,24 +345,25 @@ static int vda_h264_end_frame(AVCodecContext *avctx)
 
     CFRelease(coded_frame);
 
+    if (!vda->frame)
+        return AVERROR_UNKNOWN;
+
     if (status != kVDADecoderNoErr) {
         av_log(avctx, AV_LOG_ERROR, "Failed to decode frame (%d)\n", status);
         return AVERROR_UNKNOWN;
     }
 
-    if (vda->frame) {
-        av_buffer_unref(&frame->buf[0]);
+    av_buffer_unref(&frame->buf[0]);
 
-        frame->buf[0] = av_buffer_create((uint8_t*)vda->frame,
-                                         sizeof(vda->frame),
-                                         release_buffer, NULL,
-                                         AV_BUFFER_FLAG_READONLY);
-        if (!frame->buf)
-            return AVERROR(ENOMEM);
+    frame->buf[0] = av_buffer_create((uint8_t*)vda->frame,
+                                     sizeof(vda->frame),
+                                     release_buffer, NULL,
+                                     AV_BUFFER_FLAG_READONLY);
+    if (!frame->buf)
+        return AVERROR(ENOMEM);
 
-        frame->data[3] = (uint8_t*)vda->frame;
-        vda->frame = NULL;
-    }
+    frame->data[3] = (uint8_t*)vda->frame;
+    vda->frame = NULL;
 
     return 0;
 }