bgpd: Actually find the sequence number for `bgp extcommunity-list...`

bgpd: Actually find the sequence number for `bgp extcommunity-list...`

The code in the bgp extcommunity-list function was using
argv_find to get the correct idx.  The problem was that
we had already done argv_finds before and idx was non-zero
thus having us always set the seq pointer to what was last
looked up.  This causes us to pass in a value to the
underlying function and it would just wisely ignore it
causing a seq number of 0.

We would then write this seq number of 0 and then immediately
reject it on read in again.  BOO!

Actually handle argv_find the way it was meant to be.

Ticket:CM-29926
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>