Fix Unknown read in pcpp::NflogLayer::parseNextLayer

[sashashura]

Closes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=59631

[codecov]

Codecov Report

Merging #1178 (f2e9f33) into dev (843765f) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev    #1178      +/-   ##
==========================================
- Coverage   82.11%   82.10%   -0.01%     
==========================================
  Files         156      157       +1     
  Lines       20093    20098       +5     
  Branches     7575     7575              
==========================================
+ Hits        16499    16502       +3     
+ Misses       3080     3074       -6     
- Partials      514      522       +8     

Flag	Coverage Δ
alpine315	68.54% <0.00%> (-0.01%)	⬇️
centos7	71.93% <66.66%> (+0.03%)	⬆️
fedora34	68.37% <0.00%> (-0.01%)	⬇️
macos-11	60.12% <100.00%> (-0.01%)	⬇️
macos-12	60.17% <100.00%> (-0.01%)	⬇️
macos-ventura	60.12% <100.00%> (+<0.01%)	⬆️
mingw32	68.78% <0.00%> (-0.05%)	⬇️
mingw64	68.82% <0.00%> (+<0.01%)	⬆️
npcap	82.14% <100.00%> (-0.04%)	⬇️
ubuntu1804	72.16% <66.66%> (+0.02%)	⬆️
ubuntu2004	69.19% <0.00%> (-0.02%)	⬇️
ubuntu2204	68.13% <0.00%> (-0.01%)	⬇️
ubuntu2204-icpx	59.17% <100.00%> (+<0.01%)	⬆️
unittest	82.10% <100.00%> (-0.01%)	⬇️
windows-2019	82.18% <100.00%> (-0.01%)	⬇️
windows-2022	82.19% <100.00%> (-0.01%)	⬇️
winpcap	82.15% <100.00%> (-0.01%)	⬇️
zstd	71.97% <100.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
Common++/header/GeneralUtils.h	100.00% <100.00%> (ø)
Packet++/header/NflogLayer.h	96.00% <100.00%> (-0.16%)	⬇️

... and 6 files with indirect coverage changes