Fixed ESP padding in decryption

[ppachocki]

This PR corrects the value of the padding field of the _ESPPlain instance returned by decrypt method in CryptAlgo class.

[codecov]

Codecov Report

Merging #4123 (4c9568f) into master (63838a3) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4123      +/-   ##
==========================================
- Coverage   81.95%   81.94%   -0.01%     
==========================================
  Files         330      330              
  Lines       76200    76200              
==========================================
- Hits        62449    62443       -6     
- Misses      13751    13757       +6     

Files Changed	Coverage Δ
scapy/layers/ipsec.py	88.80% <100.00%> (ø)

... and 9 files with indirect coverage changes

[p-l-]

HI there, thanks for your PR. Could you add a test that is broken before your PR and fixed with it? Also, it would greatly help the maintainers if you could provide explanations about your change. Thanks!

[gpotter2]

The fix looks right altough a test would be nice.