bgpd: Assign large enough data structure for coverity to be happy

bgpd: Assign large enough data structure for coverity to be happy

When implementing the bgp_packet_mpunreach_prefix a uint8_t array
of 3 bytes was created and then assigned to a label type, which
is 4 bytes and then various pointer work is done on it.  Eventually
coverity is complaining that the 3 -vs- 4 bytes is not enough
to properly dereference it.  Just make the uint8_t 4 bytes
and be done with it.

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