Use `file_exists` to verify that config file exists

There might be the case that `fopen($file, 'r')` returns false and thus ownCloud might believe that the config file is empty and thus potentially leading to an overwrite of the config file.

This changeset introduces `file_exists` again which was used in ownCloud 5 where no such problems where reported and should not be affected by such problems.

Ref https://github.com/owncloud/core/issues/12785#issuecomment-71548720

lib/private/config.php

@@ -166,7 +166,9 @@ class Config {
 		// Include file and merge config
 		foreach ($configFiles as $file) {
 			$filePointer = @fopen($file, 'r');
-			if($file === $this->configFilePath && $filePointer === false) {
+			if($file === $this->configFilePath &&
+				$filePointer === false &&
+				@!file_exists($this->configFilePath)) {
 				// Opening the main config might not be possible, e.g. if the wrong
 				// permissions are set (likely on a new installation)
 				continue;