Avoid PHP fatal error

After last change to file rcube_ldap.php, my roundcube instance was getting this error:

PHP Fatal error:  Cannot use object of type Net_LDAP3_Result as array in ...

In

php
protected function extended_search($count = false)

$result = $this->ldap->search() returns a LDAP object (whatever package we use).
If the search returns no results (and if $is_extended_search is false), then it gets to line 971 trying to do a usort() and then a count() on an object, instead of an array.