bgpd: Rollback some of the changes made for invalid AS_PATH segment fix

bgpd: Rollback some of the changes made for invalid AS_PATH segment fix

Some of the changes made in commit cddb8112b80fa9867156c637d63e6e79eeac67bb
don't work particularly well for other changes that need to be made to
address BGP attribute error handling problems. In particular, returning
a pointer from complex attribute data parsing functions will not suffice
to express the require range of return status conditions.

* bgp_aspath.c: (assegments_parse) Rollback to a more minimal set of
  changes to fix the original problem.
  (aspath_parse) Slightly needless pushing around of code, and taking
  2 parameters to say whether ot use 2 or 4 byte encoding seems unnecessary.
* bgp_attr.c: (bgp_attr_as{,4}path) Rollback, in preparation for BGP
  attribute error handling update.