bgpd: Fix occassional turn off of extended-nexthop for an if

bgpd: Fix occassional turn off of extended-nexthop for an if

Sometimes, like once every 400 iterations, when you restart
Quagga, extended-nexthop has been turned off for interface
based config( for 5549 ).

Examining the code, there is only 1 real path to setting
the PEER_FLAG_CAPABILITITY_ENHE and that is through
peer_conf_interface_get.  Modify this code path
to always set the PEER_FLAG_CAPABILITY_ENHE if it is
not already set.

In addition, fix a possible pointer dereference.

Ticket: CM-12929
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>