bgpd: When calling bgp_process, prevent infinite loop

bgpd: When calling bgp_process, prevent infinite loop

If we have this construct:

for (pi = bgp_dest_get_bgp_path_info(dest); pi; pi = pi->next) {
     ...
     bgp_process();
}

This can induce an infinite loop.  This happens because bgp_process
will move the unsorted items to the top of the list for handling,
as such it is necessary to hold the next pointer to the side
to actually look at each possible bgp_path_info.

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