Make page validation stricter

page_simple_validate_old(), page_simple_validate_new():
Require PAGE_N_DIR_SLOTS to be at least 2.

storage/innobase/page/page0page.cc

@@ -2020,10 +2020,9 @@ page_simple_validate_old(
 
 	n_slots = page_dir_get_n_slots(page);
 
-	if (UNIV_UNLIKELY(n_slots > UNIV_PAGE_SIZE / 4)) {
-		ib::error() << "Nonsensical number " << n_slots
-			<< " of page dir slots";
-
+	if (UNIV_UNLIKELY(n_slots < 2 || n_slots > UNIV_PAGE_SIZE / 4)) {
+		ib::error() << "Nonsensical number of page dir slots: "
+			    << n_slots;
 		goto func_exit;
 	}
 
@@ -2220,10 +2219,9 @@ page_simple_validate_new(
 
 	n_slots = page_dir_get_n_slots(page);
 
-	if (UNIV_UNLIKELY(n_slots > UNIV_PAGE_SIZE / 4)) {
-		ib::error() << "Nonsensical number " << n_slots
-			<< " of page dir slots";
-
+	if (UNIV_UNLIKELY(n_slots < 2 || n_slots > srv_page_size / 4)) {
+		ib::error() << "Nonsensical number of page dir slots: "
+			    << n_slots;
 		goto func_exit;
 	}