avcodec/zmbv: obtain frame later

The frame is not needed that early so obtaining it later avoids
the costly operation in case other checks fail.

Fixes: Timeout (14sec -> 4sec)
Fixes: 13140/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ZMBV_fuzzer-5738330308739072

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Tomas Härdin <tjoppen@acc.umu.se>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 177b40890c)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/zmbv.c

@@ -519,9 +519,6 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *got_frame, AVPac
         return AVERROR_INVALIDDATA;
     }
 
-    if ((ret = ff_get_buffer(avctx, frame, 0)) < 0)
-        return ret;
-
     if (c->comp == 0) { // uncompressed data
         if (c->decomp_size < len) {
             av_log(avctx, AV_LOG_ERROR, "Buffer too small\n");
@@ -547,6 +544,9 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *got_frame, AVPac
         av_log(avctx, AV_LOG_ERROR, "decompressed size %d is incorrect, expected %d\n", c->decomp_len, expected_size);
         return AVERROR_INVALIDDATA;
     }
+    if ((ret = ff_get_buffer(avctx, frame, 0)) < 0)
+        return ret;
+
     if (c->flags & ZMBV_KEYFRAME) {
         frame->key_frame = 1;
         frame->pict_type = AV_PICTURE_TYPE_I;