myisam_repair_threads > 1 causes crash

Problem: parallel repair (myisam_repair_threads > 1) of a myisam
table with two or more fulltext keys that use the same parser may
lead to a server crash. ALTER TABLE ENABLE KEYS is affected as well.

Fix: properly initialize fulltext structures for parallel repair.

Note: 1. there's no deterministic test case.
2. now we call parser->init() for each fulltext key
(not for each fulltext parser used).