virdomainjob: check if cb is not NULL before dereferencing
virdomainjob: check if cb is not NULL before dereferencing

The callback struct does not always have to be set which could
cause a dereferencing of a NULL pointer. This patch adds check
against NULL in missing places before dereferencing.

Signed-off-by: Kristina Hanicova <e70d889c982bbe3b9ce2841af457ca911bff1baa@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Signed-off-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

jfehlig/libvirt,zippy2/libvirt,olafhering/libvirt,olafhering/libvirt,zippy2/libvirt,libvirt/libvirt,jfehlig/libvirt,jfehlig/libvirt,jfehlig/libvirt,libvirt/libvirt,olafhering/libvirt,libvirt/libvirt,zippy2/libvirt,zippy2/libvirt,libvirt/libvirt,olafhering/libvirt