Fix type issue in ngx_http_auth_ldap_ssl_handshake

nyoxi's previous commits looked like they would fix the issue with nginx
minions crashing when the session with the ldap server broke but it wasn't
compiling on my system. However when I attempted to compile them into nginx
1.7.1 was resulting in a type error. This seems to fix that problem?

I am not an expert in C, if I horribly broke some critical part I'm sorry XD