Handle other retransmit case in DTLSTransport

[sgfn]

Closes #137

This isn't really a fix per se, but at least DTLSTransport won't crash anymore

[codecov]

Codecov Report

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

Project coverage is 88.30%. Comparing base (15eee61) to head (bccbc7f).

Files	Patch %	Lines
lib/ex_webrtc/dtls_transport.ex	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #148      +/-   ##
==========================================
- Coverage   88.40%   88.30%   -0.10%     
==========================================
  Files          36       36              
  Lines        1897     1899       +2     
==========================================
  Hits         1677     1677              
- Misses        220      222       +2     

Files	Coverage Δ
lib/ex_webrtc/dtls_transport.ex	85.12% <0.00%> (-1.44%)	⬇️

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