bgpd: CVE-2011-3327 (ext. comm. buffer overflow)

bgpd: CVE-2011-3327 (ext. comm. buffer overflow)

This vulnerability (CERT-FI #513254) was reported by CROSS project.
They have also suggested a fix to the problem, which was found
acceptable.

The problem occurs when bgpd receives an UPDATE message containing
255 unknown AS_PATH attributes in Path Attribute Extended Communities.
This causes a buffer overlow in bgpd.

* bgp_ecommunity.c
  * ecommunity_ecom2str(): perform size check earlier