bgpd/security: CVE-2010-1674 Fix crash due to extended-community parser error

bgpd/security: CVE-2010-1674 Fix crash due to extended-community parser error

* bgp_attr.c: (bgp_attr_ext_communities) Certain extended-community attrs
  can leave attr->flag indicating ext-community is present, even though no
  extended-community object has been attached to the attr structure.  Thus a
  null-pointer dereference can occur later.
  (bgp_attr_community) No bug fixed here, but tidy up flow so it has same
  form as previous.

  Problem and fix thanks to anonymous reporter.
(cherry picked from commit 0c46638122f10019a12ae9668aec91691cf2e017)