DP: add NULL check to be_ptask_{enable|disable}
Currently the files and the proxy provider do not provide a check online
method (DPM_CHECK_ONLINE). The files provider because it can never go
offline. The proxy provider because there is no generic way to check
since the nature of the actual provider is unknown.
Since the method is missing check_if_online() jumps into the error
handling block were we try to reset the offline state
unconditionally. If there is no check_if_online_ptask, which never
exists for the files provider and will not be available in the proxy
provider as long as the backend is online, be_ptask_{enable|disable}
will be called with a NULL pointer.
Related to https://pagure.io/SSSD/sssd/issue/4014
Reviewed-by: Tomas Halman <thalman@redhat.com>
Reviewed-by: Alexey Tikhonov <atikhono@redhat.com>