MDEV-21608 Assertion `n_ext == dtuple_get_n_ext(dtuple)' failed during updation of PK

- n_ext value may be less than dtuple_get_n_ext(dtuple) when PK is being
updated and new record inherits the externally stored fields from
delete mark old record.

mysql-test/suite/innodb/r/innodb-blob.result

@@ -153,3 +153,9 @@ a
 SELECT * FROM t2;
 a
 DROP TABLE t1,t2,t3;
+CREATE TABLE t1(f1 INT PRIMARY KEY, f2 TEXT)ENGINE=InnoDB
+ROW_FORMAT=REDUNDANT;
+SET @fill_amount = (@@innodb_page_size / 2 ) + 1 ;
+INSERT INTO t1 VALUES(1, REPEAT(2, @fill_amount));
+UPDATE t1 SET f1 = 2;
+DROP TABLE t1;

mysql-test/suite/innodb/t/innodb-blob.test

@@ -206,3 +206,10 @@ SELECT a FROM t3;
 SELECT * FROM t2;
 
 DROP TABLE t1,t2,t3;
+
+CREATE TABLE t1(f1 INT PRIMARY KEY, f2 TEXT)ENGINE=InnoDB
+		ROW_FORMAT=REDUNDANT;
+SET @fill_amount = (@@innodb_page_size / 2 ) + 1 ;
+INSERT INTO t1 VALUES(1, REPEAT(2, @fill_amount));
+UPDATE t1 SET f1 = 2;
+DROP TABLE t1;

storage/innobase/include/rem0rec.ic

@@ -1626,7 +1626,11 @@ rec_get_converted_size(
 
 	data_size = dtuple_get_data_size(dtuple, 0);
 
-	ut_ad(n_ext == dtuple_get_n_ext(dtuple));
+	/* If primary key is being updated then the new record inherits
+	externally stored fields from the delete-marked old record.
+	In that case, n_ext may be less value than
+	dtuple_get_n_ext(tuple). */
+	ut_ad(n_ext <= dtuple_get_n_ext(dtuple));
 	extra_size = rec_get_converted_extra_size(
 		data_size, dtuple_get_n_fields(dtuple), n_ext);