lavc/videotoolbox_vp9: fix vpcC flags offset

Write the 24-bit vpcC flags field at the current cursor position after
the version byte. The previous code wrote to p+1 instead of p, leaving
one byte uninitialized between version and flags and shifting all
subsequent fields (profile, level, bitdepth, etc.) by one byte.

Signed-off-by: Jun Zhao <barryjzhao@tencent.com>
(cherry picked from commit 19c4bcae68a16ea893b0558d720dd4a2ecd58f0f)

libavcodec/videotoolbox_vp9.c

@@ -83,7 +83,7 @@ CFDataRef ff_videotoolbox_vpcc_extradata_create(AVCodecContext *avctx)
     p = vt_extradata;
 
     *p++ = 1; /* version */
-    AV_WB24(p + 1, 0); /* flags */
+    AV_WB24(p, 0); /* flags */
     p += 3;
 
    *p++ = h->h.profile;