Fix unpack() offset error naming a nonexistent parameter

[lacatoire]

The unpack() error for an out-of-range $offset names a parameter that does not exist. The signature is unpack(string $format, string $string, int $offset = 0), and $data was renamed to $string in 25f1c40.

try { unpack("N", "abcd", 9); } catch (ValueError $e) { echo $e->getMessage(); }

Before:

unpack(): Argument #3 ($offset) must be contained in argument #2 ($data)

After:

unpack(): Argument #3 ($offset) must be contained in argument #2 ($string)

unpack_offset.phpt is updated, and now also lists the parameter names through Reflection so a future rename cannot leave this message stale. The test passes.