Fixed typo ensuring header str is \0 terminated

This was obviously intended to send an empty header if
php_gmtime_r() fails. Currently it could send garbage.
Merge branch 'PHP-5.4' into PHP-5.5

* PHP-5.4:
  Fixed typo ensuring header str is \0 terminated
Merge branch 'PHP-5.5'

* PHP-5.5:
  Fixed typo ensuring header str is \0 terminated