avcodec/hevc: keep the active VPS when clearing a layer's SPS

set_sps() with a NULL SPS released s->vps before returning, also when only
a non-base layer was cleared while the base layer keeps decoding. The next
base-layer frame then dereferenced the NULL VPS in ff_hevc_is_alpha_video().
Release s->vps only when a new SPS is installed alongside it.

Assisted-by: Fairy
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/hevc/hevcdec.c

@@ -768,6 +768,8 @@ static int set_sps(HEVCContext *s, HEVCLayerContext *l, const HEVCSPS *sps)
     if (!sps)
         return 0;
 
+    av_refstruct_unref(&s->vps);
+
     ret = pic_arrays_init(l, sps);
     if (ret < 0)
         goto fail;