HPCC-9115 Fedora389 LDAP returns NULL errstring

On Fedora, the call to
   ldap_get_option(user_ld, LDAP_OPT_ERROR_STRING, &ldap_errstring);
sets the string pointer to NULL, causing exception to be thrown when
later dereferenced. This fix checks for NULL before deref

Signed-off-by: William Whitehead <william.whitehead@lexisnexis.com>
Merge pull request #4212 from RussWhitehead/fedoraFix

HPCC-9115 Fedora389 LDAP returns NULL errstring

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>