virDomainMomentAssignDef: Don't dereference a NULL pointer

virDomainMomentAssignDef: Don't dereference a NULL pointer

This functions tries to add a domain moment (love the name!) onto
a list of domain moments. Firstly, it checks if another moment
with the same name already exists. Then, it creates an empty
moment (without initializing its definition) and tries to add the
moment onto the list dereferencing moment definition in that
process. If it succeeds (which it never can), only after that it
sets moment->def.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>