Deallocate LDAP connections in safe way.
    
    Current code does isc_mem_detach() before isc_mem_put() so
    memory context reference counter is wrong for a small time frame.
    
    Macro MEM_PUT_AND_DETACH will handle it properly including
    pointer zeroing.
    
    Signed-off-by: Petr Spacek <pspacek@redhat.com>