conf: snapshot: check return value of virDomainSnapshotObjListNum

If it's negative, this might result in a request to allocate lots of
memory.

src/conf/snapshot_conf.c

@@ -1026,7 +1026,7 @@ virDomainListSnapshots(virDomainSnapshotObjListPtr snapshots,
     int ret = -1;
     int i;
 
-    if (!snaps)
+    if (!snaps || count < 0)
         return count;
     if (VIR_ALLOC_N(names, count) < 0 ||
         VIR_ALLOC_N(list, count + 1) < 0) {