MDEV-23566 SIGSEGV in mtr_t::init

mtr_t::init(): Correct the debug assertion that was added in
commit c893668 (MDEV-22970).
When the original failure scenario involves the system tablespace,
we could have m_user_space==nullptr. Let us compare m_user_space_id
instead.

storage/innobase/include/mtr0log.h

@@ -513,7 +513,8 @@ inline void mtr_t::init(buf_block_t *b)
 {
   if (UNIV_LIKELY_NULL(m_freed_pages))
   {
-    ut_ad(m_user_space->id == b->page.id().space());
+    ut_ad(m_log_mode != MTR_LOG_ALL ||
+          m_user_space_id == b->page.id().space());
     if (m_freed_pages->remove_if_exists(b->page.id().page_no()) &&
         m_freed_pages->empty())
     {