TO TERMINATE SERVER.

Problem:
Using a large amount of nested sub-expressions in boolean full-text
search (FTS) can cause a crash.

Analysis:
For each nested sub-expression, we call fts_ast_visit_sub_exp().
which is called recursively until it reaches to the last
sub-expression.
when we use large number of nested sub-expression in FTS. Because of
undefined base condition, recursive calls to fts_ast_visit_sub_exp()
aborts the server with stack overflow error.

Fix:
Added a limit on number of nested arguments should be allowed in
fulltext search.

RB: 23443
Reviewed by : Rahul Agarkar <rahul.agarkar@oracle.com>