Fix highlighting crash when line index is outdated

[omarelladen]

A random crash happened to me:

Micro encountered an error: runtime.boundsError runtime error: index out of range [197] with length 197
runtime/panic.go:115 (0x55a67689b1f4)
github.com/zyedidia/micro/internal/buffer/line_array.go:394 (0x55a676cf4c32)
github.com/zyedidia/micro/internal/buffer/buffer.go:1423 (0x55a676d12c88)
github.com/zyedidia/micro/internal/display/bufwindow.go:494 (0x55a676d12c3f)
github.com/zyedidia/micro/internal/display/bufwindow.go:756 (0x55a676d11833)
github.com/zyedidia/micro/internal/display/bufwindow.go:827 (0x55a676d14319)
github.com/zyedidia/micro/cmd/micro/micro.go:413 (0x55a676d73db5)
github.com/zyedidia/micro/cmd/micro/micro.go:400 (0x55a676d73b30)
internal/runtime/atomic/types.go:194 (0x55a6768a053d)
runtime/asm_amd64.s:1700 (0x55a6768dc0c1)

If you can reproduce this error, please report it at https://github.com/zyedidia/micro/issues

Since I was using version 2.0.14-1+b6 from Debian, the problem is in this line:

micro/internal/buffer/line_array.go

Line 389 in 77de728

if la.lines[lineN].search == nil {

(check https://sources.debian.org/src/micro/2.0.14-1/internal/buffer/line_array.go#L394)

I could not reproduce the bug, but it seems to me that the highlighter was trying to access a line that was just deleted, using an outdated Y position of the window. So this proposed fix is simple, but the concurrent access problem can be deeper.