Robustness: safer gzinflate/zlib usage

Relates to https://github.com/shaarli/Shaarli/pull/846

PHP's `gzinflate()` fails with an error when being passed an empty string

See:
- https://bugs.php.net/bug.php?id=71395

Signed-off-by: VirtualTam <virtualtam@flibidi.net>

application/FileUtils.php

@@ -50,7 +50,8 @@ public static function writeFlatDB($file, $content)
 
     /**
      * Read data from a file containing Shaarli database format content.
-     * If the file isn't readable or doesn't exists, default data will be returned.
+     *
+     * If the file isn't readable or doesn't exist, default data will be returned.
      *
      * @param string $file    File path.
      * @param mixed  $default The default value to return if the file isn't readable.
@@ -61,16 +62,21 @@ public static function readFlatDB($file, $default = null)
     {
         // Note that gzinflate is faster than gzuncompress.
         // See: http://www.php.net/manual/en/function.gzdeflate.php#96439
-        if (is_readable($file)) {
+        if (! is_readable($file)) {
+            return $default;
+        }
+
+        $data = file_get_contents($file);
+        if ($data == '') {
+            return $default;
+        }
+
         return unserialize(
             gzinflate(
                 base64_decode(
-                        substr(file_get_contents($file), strlen(self::$phpPrefix), -strlen(self::$phpSuffix))
+                    substr($data, strlen(self::$phpPrefix), -strlen(self::$phpSuffix))
                 )
             )
         );
     }
-
-        return $default;
-    }
 }