fts_query_t::nested_sub_exp: Keep track of nested
fts_ast_visit_sub_exp() calls.

fts_ast_visit_sub_exp(): Return DB_OUT_OF_MEMORY if the
maximum recursion depth is exceeded.

This is motivated by a change in MySQL 5.6.50:
mysql/mysql-server@e2a46b4834a0030e5380d45789772cb6beab6e92
Bug #29929684 USING MANY NESTED ARGUMENTS WITH BOOLEAN FTS CAN LEAD
TO TERMINATE SERVER