When rewriting community_writebuf() the handling of non-transitive

When rewriting community_writebuf() the handling of non-transitive
ext-communities was put into the wrong place in the loop finding
start, end and number of communities to dump. As a result the end
pointer for regular communities can point at an ext-community and
with that the COMMUNITY attribute written includes unexpected extra
bytes. This in turn causes the peer to send a NOTIFICATION error
and to terminate the session.

Fix for -portable issue #64 reported by Pier Carlo Chiodi (pierky)
OK tb@