add a check for disk_free_space in Config.php

Signed-off-by: Simon L <szaimen@e.mail.de>

lib/private/Config.php

@@ -286,10 +286,12 @@ class Config {
 		}
 
 		// Never write file back if disk space should be too low
-		$df = disk_free_space($this->configDir);
-		$size = strlen($content) + 10240;
-		if ($df !== false && $df < (float)$size) {
-			throw new \Exception($this->configDir . " does not have enough space for writing the config file! Not writing it back!");
+		if (function_exists('disk_free_space')) {
+			$df = disk_free_space($this->configDir);
+			$size = strlen($content) + 10240;
+			if ($df !== false && $df < (float)$size) {
+				throw new \Exception($this->configDir . " does not have enough space for writing the config file! Not writing it back!");
+			}
 		}
 
 		// Try to acquire a file lock