bgpd: fix Prefix-SID parse error

bgpd: fix Prefix-SID parse error

Prefix-SID is desined to capable for TLV array.
That behaviour is important to support SR-MPLS feature
and that supported by previous PR #5418.

In that implementation, but if some additional data
(such as next BGP update message or next path attributes)
was present after Prefix-SID path attribute,
bgpd will parse that addional data as Prefix-SID TLV.

This commit fix that. before this commit, loop condition
is determed by stream is readable or not. In more correct
implementatoin, the prefix-sid boundaly should be checked
additonally. the length of Prefix-sid path attribute can
be get by bgp_attr_parse_args.

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