fix: cannot read property hdlr of undefined

[xiaoyuhen]

fix: cannot read property hdlr of undefined

Description

In some case, the hdlr box is not the first sub box of trak box.

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also note any relevant details for your test configuration.

• this video can play normaly

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 #66 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #66   +/-   ##
=======================================
  Coverage   51.65%   51.65%           
=======================================
  Files         123      123           
  Lines        1717     1717           
=======================================
  Hits          887      887           
  Misses        830      830

Impacted Files	Coverage Δ
packages/griffith-mp4/src/mp4/mp4Box.js	88.88% <100%> (ø)	⬆️

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