Fix out of bounds access in quartiles

[Zentrik]

For an example consider len=2, h1_end underflows and h2_begin = 2.

[Kobzol]

Hi, did you encounter a crash locally with the original code? 🤔

[Zentrik]

Yes when I set NUM_PREVIOUS_COMMITS to 3, I assume pcs then had length 2 and so there was a panic on let q1 = median(&pcs[..=h1_end]);

[Kobzol]

Hmm, yeah, but the code uses NUM_PREVIOUS_COMMITS = 30, so it doesn't really have to deal with this. That being said, the change is simple enough, so why not :)

[Kobzol]

Could you please just squash the commits into a single one? (Let me know if you want help with that).

[Zentrik]

I presume if len=0 this will still crash actually, I could add some code to in that case default to the DEFAULT_SIGNIFICANCE_THRESHOLD if you want.

[Kobzol]

No, that's fine. Thanks!