Is it safe to tie a horse by the reins of its bridle?

No it is not safe. Because if the horse pulls back when its tied up with reins attached to the bit in its mouth it can do severe damage to its teeth, tongue, jaw & rest of its mouth. It would be far safer for you to put a halter on over/under the bridle & have it tied up from that - preferably with a panic-snap (or quick release) lead.