spider_db_mbase_util::open_item_func() is a monster function.
It is difficult to maintain while it is expected that we need to
modify it when a new SQL function or a new func_type is added.

We split the function into two distinct functions: one handles the
case of str != NULL and the other handles the case of str == NULL.

This refactoring was done in a conservative way because we do not
have comprehensive tests on the function.

It also fixes a problem (MDEV-29447) where field items that are
arguments of a func item may be used before created / initialised.

Note this commit is a port of
3836098c (MDEV-26285) to current
versions 10.3+.
Signed-off-by: Yuchen Pei <yuchen.pei@mariadb.com>