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.

program/lib/Roundcube/rcube_ldap.php

@@ -921,7 +921,7 @@ class rcube_ldap extends rcube_addressbook
         $result = $this->ldap->search($base_dn, $prop['filter'], $prop['scope'], $attrs, $prop, $count);
 
         // we have a search result resource, get all entries
-        if (!$count && $result && $result->count() > 0) {
+        if (!$count && $result) {
             $result = $result->entries();
             unset($result['count']);
         }