Coverity: fix REVERSE_INULL for pevent->inst
    
    With the DynDB API changes, the ldap instance is acquired
    differently. Previously, obtaining the instance could fail when
    LDAP was disconnecting, thus the NULL check was necessary in the
    cleanup part.
    
    Now, inst is obtained directly from the API. I'm not sure what is
    the exact behaviour in edge cases such as LDAP disconnecting, so
    I perform the NULL check a bit earlier, just to be safe.
    
    Reviewed-By: Petr Menšík <pemensik@redhat.com>