bgpd: Treat empty reachable NLRI as a EOR

bgpd: Treat empty reachable NLRI as a EOR

This issue was discovered on a live session with an extremely
old cisco 7206VXR router running 12.2(33)SRE4.  The sending router
is sending us an empty NLRI that is MP_REACH.  From RFC
exploration(thanks Russ!) it appears that this was
considered a 'valid' way to send EOR.

Following discussion decided that we should treat
this situation as a EOR marker instead of bringing
down the session.

Applying this fix on the FRR router seeing this issue
allows it to continue it's peering relationship with
the ASR.  Since this is a point fix I do not see
a high likelihood of further fallout.

Fixes: #1258
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>