fix: cannot read property start of undefined

[xiaoyuhen]

fix: cannot read property start of undefined

Description

close #46

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

[codecov-io]

Codecov Report

Merging #86 into master will decrease coverage by 0.28%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
- Coverage   50.76%   50.48%   -0.29%     
==========================================
  Files         124      124              
  Lines        1759     1769      +10     
==========================================
  Hits          893      893              
- Misses        866      876      +10

Impacted Files	Coverage Δ
.../griffith-mp4/src/mp4/utils/getFragmentPosition.js	0% <0%> (ø)	⬆️
packages/griffith-mp4/src/mp4/mp4Probe.js	0% <0%> (ø)	⬆️
packages/griffith-mp4/src/player.js	0% <0%> (ø)	⬆️
packages/griffith-mp4/src/mse/controller.js	0% <0%> (ø)	⬆️

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