luci-base: fix off-by-one in cbi_update_table()

Fixes: #5189
Fixes: f9196353 ("luci-base: batch DOM updates to prevent slowdown")
Ref: https://github.com/openwrt/luci/commit/f9196353

#commitcomment-53495580
Signed-off-by: Jo-Philipp Wich <jo@mein.io>