(crashreport) limit output chunks to 5MB, if we can't find </head> within that...

(crashreport) limit output chunks to 5MB, if we can't find </head> within that area it's very unlikely that it exists in the output.

report:
PHP Fatal error:  Allowed memory size of 268435456 bytes exhausted (tried to allocate 35997807 bytes) in /usr/local/www/csrf/csrf-magic.php on line 144

(cherry picked from commit 2f741d7a)

src/www/csrf/csrf-magic.php

@@ -410,7 +410,8 @@ if (function_exists('csrf_startup')) {
 }
 // Initialize our handler
 if ($GLOBALS['csrf']['rewrite']) {
-    ob_start('csrf_ob_handler');
+    // limit output chunks to max 5MB
+    ob_start('csrf_ob_handler', 5242880);
 }
 // Perform check
 csrf_check();