[Bug Report] Crash Log viewer when scrolling
### Version

Nightly

### Category

Major — The software does not fulfill the task it is designed for.

### Steps to Reproduce

1. Start moe
2. `:log` Open Log viewer
3. `Ctrl-j` Move to the prev window
4. `yy`, `yy` yank lines twice
5. `Ctrl-k` Move to the log viewer window
6. Scroll on the log viewer window

### Detailed Description

/home/fox/git/moe/src/moe.nim(104) moe
/home/fox/git/moe/src/moe.nim(100) main
/home/fox/git/moe/src/moepkg/mainloop.nim(404) editorMainLoop
/home/fox/git/moe/src/moepkg/editorstatus.nim(771) update
/home/fox/git/moe/src/moepkg/editorview.nim(586) seekCursor
 /home/fox/git/moe/src/moepkg/editorview.nim(268) scrollDown
/home/fox/.choosenim/toolchains/nim-1.6.14/lib/system/fatal.nim(54) sysFatal
Error: unhandled exception: value out of range: -1 notin 0 .. 9223372036854775807 [RangeDefect]