Description:
------------
PHP-5.6 crashes while processing invalid XML input with wddx_deserialize

https://github.com/php/php-src/blob/PHP-5.6/ext/wddx/wddx.c#L1170

 wddx_stack_top(&stack, (void**)&ent);
 *return_value = *(ent->data);

ent value is null but is not checked and then used to assign the return value. This doesn't happen with PHP-7.0, but the code here changed a little, I guess some of these macro check the value and prevent it from happening:

https://github.com/php/php-src/blob/PHP-7.0.9/ext/wddx/wddx.c#L1075

 wddx_stack_top(&stack, (void**)&ent);
 ZVAL_COPY(return_value, &ent->data);


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

$xml = <<< XML
<?xml version='1.0' ?>
<!DOCTYPE wddxPacket SYSTEM 'wddx_0100.dtd'>
<wddxPacket version='1.0'>
        |array>
                <var name="XXXX">
                        <boolean value="this">
                        </boolean>
                </var>
                <var name="YYYY">
                        <var name="UUUU">
                                <var name="EZEZ">
                                </var>
                        </var>
                </var>
        </array>
</wddxPacket>
XML;

$array = wddx_deserialize($xml);
var_dump($array);


Expected result:
----------------
NULL

Actual result:
--------------
operac@hp2:~/testafl$ /home/operac/build2/bin/php -n wdxnull56.php
ASAN:SIGSEGV
=================================================================
==16677==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x0000015c6577 bp 0x7ffef52c6b50 sp 0x7ffef52c6a80 T0)
    #0 0x15c6576 in php_wddx_deserialize_ex /home/operac/build2/php-src-56/ext/wddx/wddx.c:1177
    #1 0x15c7737 in zif_wddx_deserialize /home/operac/build2/php-src-56/ext/wddx/wddx.c:1383
    #2 0x1d5b393 in zend_do_fcall_common_helper_SPEC /home/operac/build2/php-src-56/Zend/zend_vm_execute.h:558
    #3 0x1c0463c in execute_ex /home/operac/build2/php-src-56/Zend/zend_vm_execute.h:363
    #4 0x194c382 in zend_execute_scripts /home/operac/build2/php-src-56/Zend/zend.c:1341
    #5 0x169a2df in php_execute_script /home/operac/build2/php-src-56/main/main.c:2613
    #6 0x1d64366 in do_cli /home/operac/build2/php-src-56/sapi/cli/php_cli.c:994
    #7 0x4550a0 in main /home/operac/build2/php-src-56/sapi/cli/php_cli.c:1378
    #8 0x7f55aeab882f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
    #9 0x4556b8 in _start (/home/operac/build2/bin/php+0x4556b8)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/operac/build2/php-src-56/ext/wddx/wddx.c:1177 php_wddx_deserialize_ex
==16677==ABORTING