detect/vlan: fix leak with bad signature

[catenacyber]

Link to ticket: https://redmine.openinfosecfoundation.org/issues/
None, found in #13319

Describe changes:

• detect/vlan: fix leak with bad signature

[inashivb]

Noice! LGTM 🚀

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.43%. Comparing base (b4095bf) to head (a493f13).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #13321      +/-   ##
==========================================
+ Coverage   83.41%   83.43%   +0.02%     
==========================================
  Files         995      995              
  Lines      272813   272814       +1     
==========================================
+ Hits       227558   227621      +63     
+ Misses      45255    45193      -62     

Flag	Coverage Δ
fuzzcorpus	62.09% <100.00%> (+0.05%)	⬆️
livemode	18.94% <0.00%> (-0.01%)	⬇️
pcap	44.90% <0.00%> (-0.02%)	⬇️
suricata-verify	65.00% <0.00%> (+0.01%)	⬆️
unittests	59.25% <0.00%> (+<0.01%)	⬆️

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[suricata-qa]

Information: QA ran without warnings.

Pipeline 26252

[victorjulien]

Merged in #13328, thanks!