Fix crash when receiving short mac

[maxammann]

Sending short mac values can lead to a crash of a WebRTC client/server.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.28%. Comparing base (aae46b7) to head (4587739).

Files with missing lines	Patch %	Lines
rtc-dtls/src/crypto/crypto_cbc.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
- Coverage   71.30%   71.28%   -0.03%     
==========================================
  Files         442      442              
  Lines       67361    67364       +3     
==========================================
- Hits        48030    48018      -12     
- Misses      19331    19346      +15     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.