Subject: Bug in ldap_parse_result function (ITS#643)

If a user is using ldap result to obtain search results one message
at a time and calls ldap_parse_result on one of those messages, i.e.
the message chain does not have an LDAP_RES_SEARCH_RESULT message
in the chain, ldap_parse_result returns a LDAP_DECODING_ERROR
instead of LDAP_NO_RESULTS_RETURNED.