Option parsing bug for LDAP::Conn.sasl_bind

rb_ldap_conn_sasl_bind appears to assume that the presence of any optional argument infers that all previous optional arguments are present and not nil.  

If, for example, the caller wished to bind with options passed into sasl_options, the caller would need to supply non-nil and non default arguments to sctrls, and cctrls as well.

This patch checks that arguments are not nil before using them.