(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)