OC_Group_Backend.php misses function groupexists

Fix http://bugs.owncloud.org/thebuggenie/owncloud/issues/oc-182

lib/group.php

@@ -255,7 +255,12 @@ class OC_Group {
 	 * @return bool
 	 */
 	public static function groupExists($gid){
-		return in_array( $gid, self::getGroups());
+		foreach(self::$_usedBackends as $backend){
+			if ($backend->groupExists($gid)){
+				return true;
+			}
+		}
+		return false;
 	}
 	
 	/**

lib/group/backend.php

@@ -82,4 +82,16 @@ abstract class OC_Group_Backend {
 	public function implementsActions($actions){
 		return (bool)($this->getSupportedActions() & $actions);
 	}
+
+	/**
+	 * check if a group exists
+	 * @param string $gid
+	 * @return bool
+	 */
+	public function groupExists($gid){
+		if(!$backend->implementsActions(OC_GROUP_BACKEND_GET_GROUPS)){
+			return false;
+		}
+		return in_array($gid, $this->getGroups());
+	}
 }