fix: avoid exception when calling :Reject on last element

[mivort]

This commit adds array length check gate before setting the cursor position on list entry removal with :Reject command. It allows to avoid the following exception which gets produced otherwise:

E5108: Error executing lua .../qf_helper.nvim/lua/qf_helper.lua:180: Cursor position outside buffer

[stevearc]

LGTM thanks!