conf: snapshot: check return value of virDomainSnapshotObjListNum
conf: snapshot: check return value of virDomainSnapshotObjListNum

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=878376

If it's negative, this might result in a request to allocate lots of
memory.
(cherry picked from commit 0361917619963d4bc6e9c8215368beaab9db2c5f)

mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2