fix: Linear DASH multiperiod label issue (#1352)

[adrums86]

Description

cherry-pick for a bug fix.

Specific Changes proposed

See: #1352

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

[codecov]

Codecov Report

Merging #1373 (de73795) into 2.x (1c93e79) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              2.x    #1373      +/-   ##
==========================================
+ Coverage   86.35%   86.37%   +0.01%     
==========================================
  Files          39       39              
  Lines        9871     9883      +12     
  Branches     2301     2305       +4     
==========================================
+ Hits         8524     8536      +12     
  Misses       1347     1347              

Impacted Files	Coverage Δ
src/dash-playlist-loader.js	90.35% <100.00%> (+0.27%)	⬆️
src/manifest.js	93.57% <100.00%> (+0.12%)	⬆️

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