MDEV-22871 fixup: Relax a debug assertion

In commit bf3c862 we introduced
an assertion that may dereference a null pointer.

This regression was caught by running the following:
./mtr --parallel=auto --suite=innodb \
--mysqld=--loose-innodb-adaptive-hash-index

The adaptive hash index is disabled by default since
commit 88cdfc5 (MDEV-20487)
and hence the problem was not caught earlier.

storage/innobase/include/btr0sea.h

@@ -303,7 +303,8 @@ struct btr_search_sys_t
   /** Get an adaptive hash index partition */
   partition *get_part(const dict_index_t &index) const
   {
-    ut_ad(index.table->space->id == index.table->space_id);
+    ut_ad(!index.table->space ||
+          index.table->space->id == index.table->space_id);
     return get_part(ulint(index.id), index.table->space_id);
   }