snapshot: Allow NULL to virDomainSnapshotObjGetDef

Doing so can simplify some callers.
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

src/conf/snapshot_conf.c

@@ -967,7 +967,7 @@ virDomainSnapshotRedefinePrep(virDomainPtr domain,
     }
 
     other = virDomainSnapshotFindByName(vm->snapshots, def->common.name);
-    otherdef = other ? virDomainSnapshotObjGetDef(other) : NULL;
+    otherdef = virDomainSnapshotObjGetDef(other);
     check_if_stolen = other && otherdef->common.dom;
     if (virDomainSnapshotRedefineValidate(def, domain->uuid, other, xmlopt,
                                           flags) < 0) {

src/conf/virdomainsnapshotobjlist.h

@@ -87,7 +87,7 @@ int virDomainListSnapshots(virDomainSnapshotObjListPtr snapshots,
 static inline virDomainSnapshotDefPtr
 virDomainSnapshotObjGetDef(virDomainMomentObjPtr obj)
 {
-    return (virDomainSnapshotDefPtr) obj->def;
+    return obj ? (virDomainSnapshotDefPtr) obj->def : NULL;
 }
 
 #endif /* LIBVIRT_VIRDOMAINSNAPSHOTOBJLIST_H */