lavc/avcodec: only allocate the encoding frame for encoders

And only when needed, i.e. for encoders using the simple API.

libavcodec/avcodec.c

@@ -180,14 +180,12 @@ int attribute_align_arg avcodec_open2(AVCodecContext *avctx, const AVCodec *code
 
     avci->buffer_frame = av_frame_alloc();
     avci->buffer_pkt = av_packet_alloc();
-    avci->es.in_frame = av_frame_alloc();
     avci->in_pkt = av_packet_alloc();
     avci->last_pkt_props = av_packet_alloc();
     avci->pkt_props = av_fifo_alloc2(1, sizeof(*avci->last_pkt_props),
                                      AV_FIFO_FLAG_AUTO_GROW);
     if (!avci->buffer_frame || !avci->buffer_pkt          ||
-        !avci->es.in_frame  || !avci->in_pkt     ||
-        !avci->last_pkt_props || !avci->pkt_props) {
+        !avci->in_pkt || !avci->last_pkt_props || !avci->pkt_props) {
         ret = AVERROR(ENOMEM);
         goto free_and_end;
     }

libavcodec/encode.c

@@ -412,6 +412,7 @@ int attribute_align_arg avcodec_receive_packet(AVCodecContext *avctx, AVPacket *
 
 int ff_encode_preinit(AVCodecContext *avctx)
 {
+    AVCodecInternal *avci = avctx->internal;
     int i;
 
     if (avctx->time_base.num <= 0 || avctx->time_base.den <= 0) {
@@ -563,5 +564,11 @@ FF_ENABLE_DEPRECATION_WARNINGS
     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) {
+        avci->es.in_frame = av_frame_alloc();
+        if (!avci->es.in_frame)
+            return AVERROR(ENOMEM);
+    }
+
     return 0;
 }