MDEV-19085: Remove a bogus debug assertion

MariaDB does support InnoDB tables with no stored columns.
(They are necessarily empty.)

mysql-test/suite/gcol/r/innodb_virtual_basic.result

@@ -1485,3 +1485,13 @@ SELECT * FROM t1;
 a	b
 	f1
 DROP TABLE t1;
+CREATE TABLE t1 (a INT GENERATED ALWAYS AS(1) VIRTUAL) ENGINE=InnoDB;
+ALTER TABLE t1 ADD b INT GENERATED ALWAYS AS (2) VIRTUAL;
+ALTER TABLE t1 ADD c INT;
+SELECT * FROM t1;
+a	b	c
+INSERT INTO t1 SET c=3;
+SELECT * FROM t1;
+a	b	c
+1	2	3
+DROP TABLE t1;

mysql-test/suite/gcol/t/innodb_virtual_basic.test

@@ -1429,3 +1429,11 @@ INSERT INTO t1(a) VALUES ('');
 ALTER TABLE t1 CHANGE COLUMN a a VARCHAR(85);
 SELECT * FROM t1;
 DROP TABLE t1;
+
+CREATE TABLE t1 (a INT GENERATED ALWAYS AS(1) VIRTUAL) ENGINE=InnoDB;
+ALTER TABLE t1 ADD b INT GENERATED ALWAYS AS (2) VIRTUAL;
+ALTER TABLE t1 ADD c INT;
+SELECT * FROM t1;
+INSERT INTO t1 SET c=3;
+SELECT * FROM t1;
+DROP TABLE t1;

storage/innobase/handler/handler0alter.cc

@@ -3022,7 +3022,6 @@ innobase_build_col_map(
 				     & Alter_inplace_info::ADD_COLUMN));
 	DBUG_ASSERT(!add_cols || dtuple_get_n_fields(add_cols)
 		    == dict_table_get_n_cols(new_table));
-	DBUG_ASSERT(table->s->stored_fields > 0);
 
 	const uint old_n_v_cols = uint(table->s->fields
 				       - table->s->stored_fields);