bgpd: Only set bgp_labels in bgp_update if we have num_labels

bgpd: Only set bgp_labels in bgp_update if we have num_labels

In the interest of speeding up code, there is no point in
attempting to see if a label is usable if the number of labels
passed in is 0.  Since that is a much much quicker test than
the bgp_is_valid_label() call, let's test that first.

Additionally, there is no point in walking the label[] array
passed in unless we are in the if statement, so move it inside.

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