snapshot: Avoid infloop during REDEFINE

Commit 55c2ab3e accidentally introduced an infinite loop while
checking whether a redefined snapshot would cause an infinite loop in
chasing its parents back to a root.  Alas, 'make check' did not catch
it, so my next patch will be a testsuite improvement that would have
hung and prevented the bug from being checked in to begin with.

Signed-off-by: Eric Blake <eblake@redhat.com>
Acked-by: Michal Privoznik <mprivozn@redhat.com>

src/conf/snapshot_conf.c

@@ -962,6 +962,7 @@ virDomainSnapshotRedefinePrep(virDomainPtr domain,
                          vm->def->name);
                 break;
             }
+            otherdef = virDomainSnapshotObjGetDef(other);
         }
     }