Fix LSP Crash in Code Fences

[dandeandean]

Description

This PR fixes the crash described in #461.
As of right now, the behavior that previously causes the crash can cause the LSP diagnostics to switch running diagnostics inside fenced areas (instead of outside see: test-zone.md).
While this is still undefined behavior, it is better than crashing the LSP client.
It seems to me that the logic behind when to check the links may need a major refactor in order to fix this bug, so in the mean time, we can add this as a quick fix.

Testing:

I tested this on the following files:
test-note.md
test-zone.md
empty.md

The Diagnostics switch

On Open

After Triggering the Bug

[dandeandean]

@tjex @Rahlir I am still investigating the root cause of this issue. In the mean time could you take a look at this? I think it may be worth closing this out so there's no crash. Let me know

[tjex]

Great, thanks for this!! There's been a lot of activity here all of sudden, and I'm a little down on free time at the moment. So just a heads up that I may take a bit to get around to this.