avcodec/encode: Fix check for encoders impl. encode-simple API

FFCodec.cb is a union.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/encode.c

@@ -589,7 +589,7 @@ int ff_encode_preinit(AVCodecContext *avctx)
     if (avctx->codec_descriptor->props & AV_CODEC_PROP_INTRA_ONLY)
         avctx->internal->intra_only_flag = AV_PKT_FLAG_KEY;
 
-    if (ffcodec(avctx->codec)->cb.encode) {
+    if (ffcodec(avctx->codec)->cb_type == FF_CODEC_CB_TYPE_ENCODE) {
         avci->in_frame = av_frame_alloc();
         if (!avci->in_frame)
             return AVERROR(ENOMEM);