BUG#39746 - Debug flag breaks struct definition

(server crash)

Altering a table with fulltext index[es] which use
pluggable fulltext parser may cause server crash
in debug builds.

The problem was that ALTER TABLE code wrongly assigned
fulltext parser name.

Also fixed that altering a table with fulltext index[es]
leave stale fulltext parser locks, which prevent
fulltext parsers from being uninstalled after
ALTER TABLE.

mysql-test/r/fulltext_plugin.result

@@ -0,0 +1,5 @@
+INSTALL PLUGIN simple_parser SONAME 'mypluglib.so';
+CREATE TABLE t1(a TEXT, b TEXT, FULLTEXT(a) WITH PARSER simple_parser);
+ALTER TABLE t1 ADD FULLTEXT(b) WITH PARSER simple_parser;
+DROP TABLE t1;
+UNINSTALL PLUGIN simple_parser;