Problem:
1. The server terminates abnormally when phrase search doesn't
filter out doc_ids correctly. This problem has been fixed in bug
2. Wrong query result: It's a regression from the bug #22709692 fix.
This fix optimize full-text search query with limit clause.
when FTS expression involves only union operation, we fetch only
number of doc_ids specified with the limit clause.
Fulltext phrase search is not an union operation and we consider
phrase search with plugin parser a union operation.
In phrase search with limit clause, we fetch limited doc_ids for
each token and if any of the selected doc_id does not contain all
tokens in correct order then we do not include that row_id in the
result set.
Therefore phrase search gets fewer number of rows than the qualified
rows exist in the table.

Fix:
Added a condition that phrase search with plugin parser is not a
union operation.

RB: 24925
Reviewed by : Annamalai Gurusami <annamalai.gurusami@oracle.com>

This is a cherry-pick of
mysql/mysql-server@5549920b7a33ef33034461d973a9ecb17ce49799
without a test case, because the test case depends on an n-gram
tokenizer that will be missing from MariaDB until MDEV-10267 is added.