ldap: Fix crash in case of empty LDAP response for CRL fetch

In case of an empty LDAP result during a CRL fetch (for example, due to
a wrong filter attribute in the LDAP URI, or invalid LDAP configuration),
the call to ldap_result2error() with NULL value for "entry" lead to
a crash.

Closes strongswan/strongswan#52.

src/libstrongswan/plugins/ldap/ldap_fetcher.c

@@ -93,8 +93,7 @@ static bool parse(LDAP *ldap, LDAPMessage *result, chunk_t *response)
 	}
 	else
 	{
-		DBG1(DBG_LIB, "finding first LDAP entry failed: %s",
-			 ldap_err2string(ldap_result2error(ldap, entry, 0)));
+		DBG1(DBG_LIB, "finding first LDAP entry failed");
 	}
 	return success;
 }