Renderer/Html/Array: Fix Undefined offset for callback fixSpaces

It avoids breaking the callback when no spaces are to be counted.

This is a fixup for 2900dd9 (#42)

Author: lazyfrosch

lib/Diff/Renderer/Html/Array.php

@@ -190,12 +190,13 @@ protected function formatLines($lines)
 	 */
 	private function fixSpaces(array $matches)
 	{
-		$spaces = $matches[1];
-		$count = strlen($spaces);
-		if($count == 0) {
+		if (count($matches) < 2) {
 			return '';
 		}
 
+		$spaces = $matches[1];
+		$count = strlen($spaces);
+
 		$div = floor($count / 2);
 		$mod = $count % 2;
 		return str_repeat('&nbsp; ', $div).str_repeat('&nbsp;', $mod);