Ensure we have enough input data before parsing date

This check have be removed in
http://git.php.net/?p=php-src.git;a=commit;h=ba2f87b50667f147c198abd31fc31eb09522f3d7
But the parser really need 17 char.
And the string need to be nul terminated for this check
So avoid reading random byte from memory.
Merge branch 'PHP-5.5' into PHP-5.6

* PHP-5.5:
  Ensure we have enough input data before parsing date
Merge branch 'PHP-5.6'

* PHP-5.6:
  Ensure we have enough input data before parsing date