Fix nexhtop group index array scaling.

The current code has the limit of 127 nexthop groups due to the
wrongly-checked bitmask_copy() return value.

PR: 254303
Reported by: Aleks <a.ivanov at veesp.com>
Approved by: re (gjb)

(cherry picked from commit 923e7f7e12670e97b097a195e69c848a6e8773a2)