avcodec/clearvideo: Only output a frame if one is coded in the packet

Fixes: Timeout (183 ms instead of about 20 sec)
Fixes: 3147/clusterfuzz-testcase-4870592182353920

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

libavcodec/clearvideo.c

@@ -290,11 +290,6 @@ static int clv_decode_frame(AVCodecContext *avctx, void *data,
     }
 
     frame_type = bytestream2_get_byte(&gb);
-    if ((ret = ff_reget_buffer(avctx, c->pic)) < 0)
-        return ret;
-
-    c->pic->key_frame = frame_type & 0x20 ? 1 : 0;
-    c->pic->pict_type = frame_type & 0x20 ? AV_PICTURE_TYPE_I : AV_PICTURE_TYPE_P;
 
     if (frame_type & 0x2) {
         if (buf_size < c->mb_width * c->mb_height) {
@@ -302,6 +297,12 @@ static int clv_decode_frame(AVCodecContext *avctx, void *data,
             return AVERROR_INVALIDDATA;
         }
 
+        if ((ret = ff_reget_buffer(avctx, c->pic)) < 0)
+            return ret;
+
+        c->pic->key_frame = frame_type & 0x20 ? 1 : 0;
+        c->pic->pict_type = frame_type & 0x20 ? AV_PICTURE_TYPE_I : AV_PICTURE_TYPE_P;
+
         bytestream2_get_be32(&gb); // frame size;
         c->ac_quant        = bytestream2_get_byte(&gb);
         c->luma_dc_quant   = 32;
@@ -323,13 +324,13 @@ static int clv_decode_frame(AVCodecContext *avctx, void *data,
                     mb_ret = ret;
             }
         }
-    } else {
-    }
 
-    if ((ret = av_frame_ref(data, c->pic)) < 0)
-        return ret;
+        if ((ret = av_frame_ref(data, c->pic)) < 0)
+            return ret;
 
-    *got_frame = 1;
+        *got_frame = 1;
+    } else {
+    }
 
     return mb_ret < 0 ? mb_ret : buf_size;
 }