TLS kex fix for curves with size not multiple of 8

[dmillescamps]

Some EC curves have size not a multiple of 8, such as secp521r1. Because of this, it will crash during the Key Exchange with a 50% chance.

Added a unit test that checks for the correct signature size, or will throw an exception before due to an "odd-length" hexadecimal string.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.90%. Comparing base (449142a) to head (5bfd82e).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4827      +/-   ##
==========================================
- Coverage   80.91%   80.90%   -0.01%     
==========================================
  Files         366      366              
  Lines       90130    90130              
==========================================
- Hits        72927    72921       -6     
- Misses      17203    17209       +6     

Files with missing lines	Coverage Δ
scapy/layers/tls/keyexchange.py	86.99% <ø> (ø)

... and 6 files with indirect coverage changes

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[guedou]

Thanks for the PR!