codec/avcodec/vaapi*: fix potential subpictures memleak

modules/codec/avcodec/vaapi.c

@@ -403,6 +403,9 @@ static vlc_va_surface_t *FindSurface( vlc_va_t *p_external, const VASurfaceID i_
              */
             p_tmp->i_refcount++;
             p_surface = p_tmp;
+            free(p_surface->subpicture);
+            p_surface->subpicture = NULL;
+            p_surface->i_sub = 0;
             vlc_mutex_unlock(&p_tmp->lock);
             break;
         }

modules/codec/avcodec/vaapi_x11.c

@@ -580,6 +580,8 @@ static int RenderDirectSubpicture(vout_display_t *vd, picture_t *picture, subpic
 {
     vout_display_sys_t *sys = vd->sys;
 
+    assert(picture->p_sys->surface->subpicture == NULL);
+
     vlc_va_subpicture_t *vasubpic = (vlc_va_subpicture_t*) calloc(i_region, sizeof(vlc_va_subpicture_t));
     if (!vasubpic)
         return VLC_EGENERIC;