Description:
------------
I've discovered a bug in the ssh2_exec function, due to using an OO wrapping library (https://github.com/Herzult/php-ssh). The library has obviously default values for parameters that are passed to the SSH2 functions, so it passes an empty array to the $env parameter when nothing is passed by the user.

I've been able to reproduce the bug with 3 lines of code, attached here in the test script.

Uncovered using:
 - PHP 7.0.6
 - ssh2 extension installed using master blob from Gitlab (https://github.com/php/pecl-networking-ssh2), using latest commit (aa1739a, Mar 25th)
 - php:7.0.6-fpm official DockerHub derived dockerfile


Test script:
---------------
<?php
        $connection = ssh2_connect('somehost.example.com', 22);
        ssh2_auth_password($connection, 'user', 'password');

        ssh2_exec($connection, 'pwd', null, []);


Expected result:
----------------
Normal execution, exit code 0

Actual result:
--------------
Segmentation fault