MDEV-22902 Assertion `!page_has_siblings(block->frame)' failed in btr_pcur_store_position

- There is a possiblity that metadata record is the only record in
the leftmost leaf page. So change the assertion to check only
previous page.

storage/innobase/btr/btr0pcur.cc

@@ -154,7 +154,7 @@ btr_pcur_store_position(
 			ut_ad(index->table->instant);
 			ut_ad(page_get_n_recs(block->frame) == 1);
 			ut_ad(page_is_leaf(block->frame));
-			ut_ad(!page_has_siblings(block->frame));
+			ut_ad(!page_has_prev(block->frame));
 			cursor->rel_pos = BTR_PCUR_AFTER_LAST_IN_TREE;
 			return;
 		}