MariaDB inherits the MySQL limitation that ALGORITHM=INPLACE cannot
create more than one FULLTEXT INDEX at a time. As part of the MDEV-11369
Instant ADD COLUMN refactoring, MariaDB 10.3.2 accidentally stopped
enforcing the restriction.

Actually, it is a bug in MySQL 5.6 and MariaDB 10.0 that an ALTER TABLE
statement with multiple ADD FULLTEXT INDEX but without explicit
ALGORITHM=INPLACE would return in an error message, rather than
executing the operation with ALGORITHM=COPY.

ha_innobase::check_if_supported_inplace_alter(): Enforce the restriction
on multiple FULLTEXT INDEX.

prepare_inplace_alter_table_dict(): Replace some code with debug
assertions. A "goto error_handled" at this point would result in
another error, because the reference count of ctx->new_table would be 0.