bgpd: Ensure we are not using AFI_MAX

bgpd: Ensure we are not using AFI_MAX

When using bgp_vty_afi_from_str it can
return AFI_MAX( but in practice never will with
our cli ).  In bgp_default_afi_safi_cmd the code
directly references:
	bgp->default_afi[afi][safi] = TRUE;

and if afi is AFI_MAX FRRR would be accessing
memory where it should not be.

Let's just provide some assurances for coverity
that this never happens.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>