bgpd: Fix AS_PATH size calculation for long paths

bgpd: Fix AS_PATH size calculation for long paths

If you have an AS_PATH with more entries than
what can be written into a single AS_SEGMENT_MAX
it needs to be broken up.  The code that noticed
that the AS_PATH needs to be broken up was not
correctly calculating the size of the resulting
message.  This patch addresses this issue.

This patch was built from an email that Andreas
sent to the dev alias for FRRouting.

Fixes: #1114
Signed-off-by: Andreas Jaggi <aj@open.ch>
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>