Description:
------------
unpack with the X format code is broken.

There are 2 related issues:

1. Unexpected warning
PHP Warning:  unpack(): Type X: outside of string

The bug is here:
File: php-src / ext / standard / pack.c

In the following snipped size is -1 for the X format code, therefore the first condition if (inputpos < size) is always true.

case 'X':
						if (inputpos < size) {
							inputpos = -size;
							i = arg - 1;		/* Break out of for loop */

							if (arg >= 0) {
								php_error_docref(NULL TSRMLS_CC, E_WARNING, "Type %c: outside of string", type);
							}
						}
						break;

2. unpack returns false and additional warning:
HP Warning:  unpack(): Type X: not enough input, need -1, have 0


Test script:
---------------
<?php

$data = pack('VV', 1, 2);

$result = unpack('Va/X' ,$data);
// PHP Warning:  unpack(): Type X: outside of string

$result = unpack('Va/X4' ,$data);
// PHP Warning:  unpack(): Type X: outside of string

$result = unpack('V1a/X4/V1b/V1c/X4/V1d', $data);
// PHP Warning:  unpack(): Type X: outside of string
// PHP Warning:  unpack(): Type X: not enough input, need -1, have 0
// unpack returns false

var_dump($result);
?>

Expected result:
----------------
No warnings.
No failures

Actual result:
--------------
Warnings:
PHP Warning:  unpack(): Type X: outside of string
PHP Warning:  unpack(): Type X: not enough input, need -1, have 0

And wrong result.