LDAP: Check the LDAP handle before using it
    
    As the connection code is async-driven, the LDAP handle might be
    invalidated before SSSD attempts to use it. Similar to commit
    5fe6ca5e339fd345119752e996c14edf8db57660, this patch adds a NULL check
    for the LDAP handle and aborts the request instead of crashing.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2305
    (cherry picked from commit 997d4bcd8e994932ac15c42524bb9f00055ee322)