bgpd: We try to skip out of updating the multipath aggregate if there are no

bgpd: We try to skip out of updating the multipath aggregate if there are no
changes in the multipath set or attributes, but failed to check for
just a bestpath change. The result is there is no attribute on the new
bestpath and we hit the assert. Added the bestpath check and
rearranged the code to only check attributes when there is no bestpath
or multipath change, so we only scan the for attribute changes when
necessary.

* bgpd/bgp_mpath.c
  * bgp_info_mpath_aggregate_update(): Added check for bestpath
    change before skipping the aggregate generation. Skip the attribute
    check if either the multipath set or bestpath has changed.