zebra: Prevent uninstall attempts when new entry is not happy

zebra: Prevent uninstall attempts when new entry is not happy

In rib_process_update_fib, the function is sent two route entries
the old ( previously installed ) and new ( the one to install )
When the function detects that the new is unusable because
the number of nexthops that are usable for that route is 0,
then we uninstall the old route.  The problem here is that
we should not attempt to uninstall any route that is
not owned by FRR.  Modify the code to not attempt
this behavior

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