Description:
------------
If a YAML file contains a pointer to a non-existing reference, the PHP interpreter crashes with segmentation fault.

I'm using the Syck library version 0.55 from http://whytheluckystiff.net/syck/.

Reproduce code:
---------------
$yaml = <<<EOS
define: &pointer_to_define
   - 1
   - 2
   - 3
reference: *pointer_to_define
EOS;

print_r(syck_load($yaml));
print "\n";

$yaml = <<<EOS
define: &pointer_to_define
   - 1
   - 2
   - 3
reference: *broken_pointer_to_define
EOS;

print_r(syck_load($yaml));
print "\n";


Expected result:
----------------
Array
(
    [define] => Array
        (
            [0] => 1
            [1] => 2
            [2] => 3
        )

    [reference] => Array
        (
            [0] => 1
            [1] => 2
            [2] => 3
        )

)

Array
(
    [define] => Array
        (
            [0] => 1
            [1] => 2
            [2] => 3
        )

    [reference] => Array
        (
            [0] => 1
            [1] => 2
            [2] => 3
        )

)



Actual result:
--------------
Array
(
    [define] => Array
        (
            [0] => 1
            [1] => 2
            [2] => 3
        )

    [reference] => Array
        (
            [0] => 1
            [1] => 2
            [2] => 3
        )

)

Segmentation fault