import: dkr - avoid NULL-pointer dereference

A malformed manifest could in principle cause a NULL pointer dereference of. Check
for this and fail early.

Fixes CID 1299642.

src/import/pull-dkr.c

@@ -864,7 +864,7 @@ static void dkr_pull_job_on_finished_v2(PullJob *j) {
                 }
 
                 e = json_variant_value(doc, "fsLayers");
-                if (!e || e->type != JSON_VARIANT_ARRAY) {
+                if (!e || e->type != JSON_VARIANT_ARRAY || e->size == 0) {
                         r = -EBADMSG;
                         goto finish;
                 }