VAAPI-X11: call cache_SubpictureRelease() only when there is a subpicture.

If creating a subpicture fails, then the picture is not going to carry a
reference to the cached subpicture structure. In this case the cached
subpicture was being freed anyway. Which resulted in a crash.

Instead check if there is an subpicture cache id and if it is > 0,
before attempting to release it.

modules/codec/avcodec/vaapi_x11.c

@@ -1345,7 +1345,8 @@ out:
             SubpictureRegionsUnlink(vd, picture);
 
         vlc_mutex_lock(&sys->cache_lock);
-        cache_SubpictureRelease(vd, picture->p_sys->i_cache);
+        if (picture->p_sys->i_cache > 0)
+            cache_SubpictureRelease(vd, picture->p_sys->i_cache);
         picture->p_sys->i_cache = 0;
         vlc_mutex_unlock(&sys->cache_lock);