Check session_create_id() input for null byte

[jorgsowa]

The function session_create_id() allows only selected characters amongst which there is no NULL byte, however the validation doesn't check for it.

Warning: session_create_id(): Prefix cannot contain special characters. Only the A-Z, a-z, 0-9, "-", and "," characters are allowed

This PR checks for a NULL byte in the function php_session_valid_key used by the PHP function session_create_id().

[Girgias]

The idea is sensible, the current implementation can be improved to take care of the issue better.

[Girgias]

I'd like to see a test where you use ini_set() to amend the session.name INI setting and have a nul byte in it.

[jorgsowa]

@Girgias, I want to cover it in the next PRs. Function session_name() doesn't share the same state with session_create_id().