nexthop: Fix off-by-one error in error path

A reference was not taken for the current nexthop entry, so do not try
to put it in the error path.

Fixes: 430a0491

 ("nexthop: Add support for nexthop groups")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>