fix: last GOP dont have audio track

[xiaoyuhen]

fix: last GOP dont have audio track

Description

if the last GOP dont have audio track, we dont need append audio sourcebuffer.

for now, if we append audio sourcebuffer, we will get this error. see #45

How Has This Been Tested?

• this video can play normal
• the example video can play normal

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 #51 into master will decrease coverage by 0.12%.
The diff coverage is 10%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
- Coverage   52.99%   52.86%   -0.13%     
==========================================
  Files         123      123              
  Lines        1672     1678       +6     
==========================================
+ Hits          886      887       +1     
- Misses        786      791       +5

Impacted Files	Coverage Δ
packages/griffith-mp4/src/mse/controller.js	0% <0%> (ø)	⬆️
.../griffith-mp4/src/mp4/utils/getFragmentPosition.js	0% <0%> (ø)	⬆️
...kages/griffith-mp4/src/mp4/utils/getBufferStart.js	80% <50%> (-2.61%)	⬇️

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