bgpd: Fix error handling when receiving BGP Prefix SID attribute

bgpd: Fix error handling when receiving BGP Prefix SID attribute

Without this patch, we always set the BGP Prefix SID attribute flag without
checking if it's malformed or not. RFC8669 says that this attribute MUST be discarded.

Also, this fixes the bgpd crash when a malformed Prefix SID attribute is received,
with malformed transitive flags and/or TLVs.

Reported-by: Iggy Frankovic <iggyfran@amazon.com>
Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>