Improve unmarshal header

[yuanchao0310]

No need to alloc empty slice if CSRC=0
Extensions always reset to zero length when unmarshal

Description

Reference issue

[yuanchao0310]

don't make empty slice can reduce bench test time.

Test result
goos: darwin
goarch: amd64
pkg: github.com/pion/rtp/v2
cpu: Intel(R) Core(TM) i7-1068NG7 CPU @ 2.30GHz

name                                      old time/op    new time/op    delta
UnmarshalHeader/NewStructWithoutCSRC-8    52.8ns ± 1%    47.7ns ± 1%  -9.67%  (p=0.008 n=5+5)
UnmarshalHeader/NewStructWithCSRC-8       68.2ns ±13%    67.6ns ±14%    ~     (p=1.000 n=5+5)

name                                      old alloc/op   new alloc/op   delta
UnmarshalHeader/NewStructWithoutCSRC-8     32.0B ± 0%     32.0B ± 0%    ~     (all equal)
UnmarshalHeader/NewStructWithCSRC-8        40.0B ± 0%     40.0B ± 0%    ~     (all equal)

name                                       old allocs/op  new allocs/op  delta
UnmarshalHeader/NewStructWithoutCSRC-8      1.00 ± 0%      1.00 ± 0%    ~     (all equal)
UnmarshalHeader/NewStructWithCSRC-8         2.00 ± 0%      2.00 ± 0%    ~     (all equal)

[yuanchao0310]

Issue #79 discuss: when CSRC empty, should we Unmarshal it as nil or []uint32{}, and choose to use []uint32{}.
but from test result, make empty slice still cost cpu time, maybe use nil is better

[yuanchao0310]

@pionbot @Sean-Der someone can help review?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.09%. Comparing base (0a6cec7) to head (a6628ed).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #188      +/-   ##
==========================================
- Coverage   91.22%   91.09%   -0.14%     
==========================================
  Files          26       26              
  Lines        3191     3188       -3     
==========================================
- Hits         2911     2904       -7     
- Misses        218      221       +3     
- Partials       62       63       +1     

Flag	Coverage Δ
go	91.09% <100.00%> (-0.14%)	⬇️
wasm	90.55% <100.00%> (-0.14%)	⬇️

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[Sean-Der]

Thank you so much for this PR @yuanchao0310

I am so sorry it took so long to merge! Totally poor management on my end, this is a great improvement.