avcodec/jvdec: Use ff_get_buffer() when the content is not reused

Fixes: Timeout (11sec -> 5sec)
Fixes: 14473/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_JV_fuzzer-5761630857592832

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

libavcodec/jvdec.c

@@ -163,13 +163,14 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
             av_log(avctx, AV_LOG_ERROR, "video size %d invalid\n", video_size);
             return AVERROR_INVALIDDATA;
         }
-        if ((ret = ff_reget_buffer(avctx, s->frame)) < 0)
-            return ret;
 
         if (video_type == 0 || video_type == 1) {
             GetBitContext gb;
             init_get_bits(&gb, buf, 8 * video_size);
 
+            if ((ret = ff_reget_buffer(avctx, s->frame)) < 0)
+                return ret;
+
             if (avctx->height/8 * (avctx->width/8) > 4 * video_size) {
                 av_log(avctx, AV_LOG_ERROR, "Insufficient input data for dimensions\n");
                 return AVERROR_INVALIDDATA;
@@ -184,6 +185,11 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
             buf += video_size;
         } else if (video_type == 2) {
             int v = *buf++;
+
+            av_frame_unref(s->frame);
+            if ((ret = ff_get_buffer(avctx, s->frame, AV_GET_BUFFER_FLAG_REF)) < 0)
+                return ret;
+
             for (j = 0; j < avctx->height; j++)
                 memset(s->frame->data[0] + j * s->frame->linesize[0],
                        v, avctx->width);