Fix possible deadlock due to missing mutex unlock in datachannel.go

[pabloFuente]

Caused by a regression introduced in commit 835ac3b#diff-b8da7f3e746d55a2fe2a4295ae5fe5cf76b9512d7665201285143e25ab3bd3b9L423

Reference issue

Fixes #3005

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 77.92%. Comparing base (49b555b) to head (778cbbd).

Files with missing lines	Patch %	Lines
datachannel.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3006      +/-   ##
==========================================
- Coverage   77.96%   77.92%   -0.05%     
==========================================
  Files          89       89              
  Lines       10578    10580       +2     
==========================================
- Hits         8247     8244       -3     
- Misses       1840     1844       +4     
- Partials      491      492       +1     

Flag	Coverage Δ
go	79.48% <0.00%> (-0.05%)	⬇️
wasm	63.68% <ø> (ø)

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