avcodec/pafvideo: Clear frame buffer later

This way the clearing can be skipped in case of some errors.

Fixes: Timeout (11sec -> 344ms)
Fixes: 14670/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PAF_VIDEO_fuzzer-5769534503387136

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

libavcodec/pafvideo.c

@@ -293,9 +293,6 @@ static int paf_video_decode(AVCodecContext *avctx, void *data,
         return ret;
 
     if (code & 0x20) {  // frame is keyframe
-        for (i = 0; i < 4; i++)
-            memset(c->frame[i], 0, c->frame_size);
-
         memset(c->pic->data[1], 0, AVPALETTE_SIZE);
         c->current_frame  = 0;
         c->pic->key_frame = 1;
@@ -332,6 +329,10 @@ static int paf_video_decode(AVCodecContext *avctx, void *data,
         c->pic->palette_has_changed = 1;
     }
 
+    if (code & 0x20)
+        for (i = 0; i < 4; i++)
+            memset(c->frame[i], 0, c->frame_size);
+
     switch (code & 0x0F) {
     case 0:
         /* Block-based motion compensation using 4x4 blocks with either