bgp_recalculate_all_bestpaths() should check if rn->info is NULL

bgp_recalculate_all_bestpaths() should check if rn->info is NULL

Signed-off-by: Daniel Walton <dwalton@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Ticket: CM-11444

The bgp table may contain nodes without an 'info' (these nodes are used
for balancing the tree, they are created by route_common() in lib/table.c).
When we call bgp_recalculate_all_bestpaths() we should avoid calling
bgp_process() for these nodes.  bgp_recalculate_all_bestpaths() is only
called when knobs are configured that could have an impact on which
routes are selected as best.