fix: level playlist parse error of ProgramDateTime

[hongfeih-es]

This PR will...

Fix level playlist parse error when #EXT-X-PROGRAM-DATE-TIME appear before #EXT-X-MAP

Why is this Pull Request needed?

When #EXT-X-PROGRAM-DATE-TIME appear before #EXT-X-MAP, rawProgramDateTime of currentInitSegment will be set:

hls.js/src/loader/m3u8-parser.ts

Line 274 in 116e7cd

frag.rawProgramDateTime = (' ' + result[5]).slice(1);

Then rawProgramDateTime of the following fragments will be set to same rawProgramDateTime:

hls.js/src/loader/m3u8-parser.ts

Lines 228 to 231 in 116e7cd

	if (currentInitSegment) {
	frag.initSegment = currentInitSegment;
	frag.rawProgramDateTime = currentInitSegment.rawProgramDateTime;
	}

Thus, programDateTime will be set to wrong value in function assignProgramDateTime:

hls.js/src/loader/m3u8-parser.ts

Lines 613 to 615 in 116e7cd

	function assignProgramDateTime(frag, prevFrag) {
	if (frag.rawProgramDateTime) {
	frag.programDateTime = Date.parse(frag.rawProgramDateTime);

So currentInitSegment.rawProgramDateTime need to be cleared after set once.

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

No.

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