crash if use 4 version of file

In many cases, it is not enough to have local, remote, and merge states. It may also be necessary to see the base state. When I send these states to vim 4, I get an error:

```
Error in decoration provider git-conflict.win:
Error executing lua: ...ite/pack/lazy/opt/git-conflict.nvim/lua/git-conflict.lua:266: Invalid 'end_row': out of range
stack traceback:
[C]: in function 'hl_range'
...ite/pack/lazy/opt/git-conflict.nvim/lua/git-conflict.lua:266: in function 'highlight_conflicts'
...ite/pack/lazy/opt/git-conflict.nvim/lua/git-conflict.lua:392: in function 'parse_buffer'
...ite/pack/lazy/opt/git-conflict.nvim/lua/git-conflict.lua:471: in function 'process'
...ite/pack/lazy/opt/git-conflict.nvim/lua/git-conflict.lua:660: in function <...ite/pack/lazy/opt/git-conflict.nvim/lua/git-conflict.lua:659>

```