Minor change in Output cache file check

Won't change anything in practice, but robuster (and faster) if ever
a cache file would be invalid
diff --git a/system/core/Output.php b/system/core/Output.php
index 9842f83..847c61e 100644
--- a/system/core/Output.php
+++ b/system/core/Output.php
@@ -552,7 +552,7 @@
 		fclose($fp);
 
 		// Strip out the embedded timestamp
-		if ( ! preg_match('/\d+TS--->/', $cache, $match))
+		if ( ! preg_match('/^\d+TS--->/', $cache, $match))
 		{
 			return FALSE;
 		}