MDEV-14126: Detect unexpected emptying of B-tree pages

If an index page becomes empty, btr_page_empty() should be called.

storage/innobase/include/page0page.ic

@@ -173,8 +173,9 @@ page_header_set_field(
 {
 	ut_ad(page);
 	ut_ad(field <= PAGE_N_RECS);
-	ut_ad(field == PAGE_N_HEAP || val < UNIV_PAGE_SIZE);
-	ut_ad(field != PAGE_N_HEAP || (val & 0x7fff) < UNIV_PAGE_SIZE);
+	ut_ad(field != PAGE_N_RECS || val);
+	ut_ad(field == PAGE_N_HEAP || val < srv_page_size);
+	ut_ad(field != PAGE_N_HEAP || (val & 0x7fff) < srv_page_size);
 
 	mach_write_to_2(page + PAGE_HEADER + field, val);
 	if (page_zip) {

storage/innobase/page/page0cur.cc

@@ -2140,6 +2140,8 @@ page_copy_rec_list_end_to_created_page(
 		rec = page_rec_get_next(rec);
 	} while (!page_rec_is_supremum(rec));
 
+	ut_ad(n_recs);
+
 	if ((slot_index > 0) && (count + 1
 				 + (PAGE_DIR_SLOT_MAX_N_OWNED + 1) / 2
 				 <= PAGE_DIR_SLOT_MAX_N_OWNED)) {

storage/innobase/page/page0page.cc

@@ -1231,6 +1231,7 @@ page_delete_rec_list_end(
 	page_header_set_field(page, NULL, PAGE_GARBAGE, size
 			      + page_header_get_field(page, PAGE_GARBAGE));
 
+	ut_ad(page_get_n_recs(page) > n_recs);
 	page_header_set_field(page, NULL, PAGE_N_RECS,
 			      (ulint)(page_get_n_recs(page) - n_recs));
 }

storage/innobase/page/page0zip.cc

@@ -4448,10 +4448,12 @@ page_zip_dir_delete(
 	slot_rec = page_zip_dir_find(page_zip, page_offset(rec));
 
 	ut_a(slot_rec);
-
+	uint16_t n_recs = page_get_n_recs(page);
+	ut_ad(n_recs);
+	ut_ad(n_recs > 1 || page_get_page_no(page) == index->page);
 	/* This could not be done before page_zip_dir_find(). */
 	page_header_set_field(page, page_zip, PAGE_N_RECS,
-			      (ulint)(page_get_n_recs(page) - 1));
+			      n_recs - 1);
 
 	if (UNIV_UNLIKELY(!free)) {
 		/* Make the last slot the start of the free list. */