shard director: handle nested workspace overflow

Propagate upwards any workspace allocation failure of the
recursive shard_param_task_l() case.

vmod/vmod_directors_shard.c

@@ -945,6 +945,9 @@ shard_param_task_l(VRT_CTX, const void *id, const char *who,
 	else
 		p->defaults = shard_param_task_l(ctx, pa, pa->vcl_name, pa);
 
+	if (p->defaults == NULL)
+		return (NULL);
+
 	return (p);
 }