MDEV-15828 Server crash or assertion `num_fts_index <= 1' failure up on ALTER...

MDEV-15828 Server crash or assertion `num_fts_index <= 1' failure up on ALTER TABLE adding two fulltext indexes

- Inplace alter shouldn't support if the number of newly added fts index
exceeds 1 even though the table undergoes rebuild. It is a regression of
MDEV-14016

mysql-test/suite/innodb_fts/r/innodb-fts-ddl.result

@@ -231,3 +231,6 @@ id	title	body
 1	MySQL Tutorial	DBMS stands for DataBase ...
 3	Optimizing MySQL	In this tutorial we will show ...
 DROP TABLE articles;
+CREATE TABLE t1 (a VARCHAR(3)) ENGINE=InnoDB;
+ALTER TABLE t1 ADD FULLTEXT KEY(a), ADD COLUMN b VARCHAR(3), ADD FULLTEXT KEY(b);
+DROP TABLE t1;

mysql-test/suite/innodb_fts/t/innodb-fts-ddl.test

@@ -271,3 +271,10 @@ SELECT * FROM articles WHERE MATCH (title, body)
 	AGAINST ('Tutorial' IN NATURAL LANGUAGE MODE);
 
 DROP TABLE articles;
+
+# Add more than one FTS index
+CREATE TABLE t1 (a VARCHAR(3)) ENGINE=InnoDB;
+ALTER TABLE t1 ADD FULLTEXT KEY(a), ADD COLUMN b VARCHAR(3), ADD FULLTEXT KEY(b);
+
+# Cleanup
+DROP TABLE t1;

storage/innobase/handler/handler0alter.cc

@@ -1193,8 +1193,10 @@ ha_innobase::check_if_supported_inplace_alter(
 				innobase_get_err_msg(
 				ER_ALTER_OPERATION_NOT_SUPPORTED_REASON_FTS);
 		}
-	} else if ((ha_alter_info->handler_flags
-		    & ALTER_ADD_NON_UNIQUE_NON_PRIM_INDEX)) {
+	}
+
+	if (ha_alter_info->handler_flags
+	    & ALTER_ADD_NON_UNIQUE_NON_PRIM_INDEX) {
 		/* ADD FULLTEXT|SPATIAL INDEX requires a lock.
 
 		We could do ADD FULLTEXT INDEX without a lock if the