script/execute: Limit the recursion depth

script/execute: Limit the recursion depth

If unbounded recursion is allowed it becomes possible to collide the
stack with the heap. As UEFI firmware often lacks guard pages this
becomes an exploitable issue as it is possible in some cases to do
a controlled overwrite of a section of this heap region with
arbitrary data.

Reported-by: B Horn <b@horn.uk>
Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>