Fix Low-Latency HLS next part selection when switching mid segment

[robwalch]

This PR will...

Fix an issue in nextPartIndex where the first independent part index is returned after newer parts have been loaded, but the next part needed is not yet listed.

Why is this Pull Request needed?

Instead of waiting for the next needed part, the player was selecting the first part, then skipping the ones that were loaded, leading to freezing of video or stalling of playback.

Are there any points in the code the reviewer needs to double check?

No

Resolves issues:

Fixes #5111

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md

[erankor]

Had to go over it a few times to understand... :)

So, essentially:

• if nextPart is positive, targetBufferTime must be >= part.start (otherwise it would break out of the loop). And, in this case, we surely don't want to load the index currently set in nextPart, because the current index i (that was already loaded) is surely a better fit
• if nextPart is already negative, this change has no effect, so that's fine as well

Looks good to me.