bgpd: allow derived router-id update if previously 0x0

bgpd: allow derived router-id update if previously 0x0

Problem found that if a router-id was not defined or derived
initially, the bgp->router_id would be set to 0x0 and used
for determining auto-rd values. When bgp received a subsequent
router-id update from zebra, bgp would not completely process
the update since it was treated as updating an already derived
router-id with a new value, which is not desired.  This also
could leave the auto rd/rt inforamation missing or invalid in
some cases.  This fix allows updating the derived router-id if
the previous value was 0/0.

Ticket: CM-31441
Signed-off-by: Don Slice <dslice@nvidia.com>