Bugfix: Low-Latency false starts when part selection does not match fragment

[robwalch]

This PR will...

Fix unused part loading when selected frag passed to _doFragLoad is not used by part selection.

Why is this Pull Request needed?

There was an expectation that _doFragLoad(frag) will resolve with the frag or a part from that frag being loaded. The result of _doFragLoad is ignored if streamController.currentFrag no longer matches frag.

This PR fixes the case where _doFragLoad(frag) resolves with a different fragment because of part selection needing to use fragmentHint or an independent part in the part list belonging to another fragment:

    if (this.loadingParts && frag.sn !== 'initSegment') {
      const partList = details.partList;
      if (partList && progressCallback) {
        if (targetBufferTime > frag.end && details.fragmentHint) {
          frag = details.fragmentHint;
        }
        const partIndex = this.getNextPart(partList, frag, targetBufferTime);
        if (partIndex > -1) {

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

Resolves issues:

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