bgpd: fix prefix-sid crash bug and add topotest (step4)

bgpd: fix prefix-sid crash bug and add topotest (step4)

This commit fix bgpd's prefix-sid type4,5 feature which has
miss implementation from https://github.com/FRRouting/frr/pull/5653
was merged. Due to some nessesary lines are not presented.

When bgpd receives multi update message with same service-sid on
prefix-sid type-5 attribute, bgpd will crash arround path-attribute's
values object reference count.

And also, this commit add a topotest to check that feature work fine.

Signed-off-by: Hiroki Shirokura <slank.dev@gmail.com>