instant_alter_column_possible(): Add the other MDEV-17459 work-around
condition. The existence of fulltext indexes only prevents instant
DROP COLUMN or changing the order of columns. Other forms of instant
ALTER TABLE are no problem.

Before commit 4e7ee166 that merged
the MDEV-18295 fix from 10.3, the work-around of MDEV-17459 in
instant_alter_column_possible() was categorically refusing any
ALGORITHM=INSTANT if any FULLTEXT INDEX was present. After that commit,
a related condition was only present in prepare_inplace_alter_table_dict()
but not in the other callers of instant_alter_column_possible().