avcodec/proresdec2: decode picture header before frame allocation

Fixes: Timeout (21sec -> 0.3sec)
Fixes: 13716/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PRORES_fuzzer-5755593333145600

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/proresdec2.c

@@ -778,9 +778,6 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
     buf += frame_hdr_size;
     buf_size -= frame_hdr_size;
 
-    if ((ret = ff_thread_get_buffer(avctx, &tframe, 0)) < 0)
-        return ret;
-
  decode_picture:
     pic_size = decode_picture_header(avctx, buf, buf_size);
     if (pic_size < 0) {
@@ -788,6 +785,10 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
         return pic_size;
     }
 
+    if (ctx->first_field)
+        if ((ret = ff_thread_get_buffer(avctx, &tframe, 0)) < 0)
+            return ret;
+
     if ((ret = decode_picture(avctx)) < 0) {
         av_log(avctx, AV_LOG_ERROR, "error decoding picture\n");
         return ret;