decode-ipv6: Set L4 proto on ipv6 incase of GRE decode error

[coledishington]

Set the L4 proto before decoding GRE in ipv6 decoding in case there is a GRE header decoding error.

Bug: #6222

• [✓] I have read the contributing guide lines at https://docs.suricata.io/en/latest/devguide/codebase/contributing/contribution-process.html
• [✓] I have signed the Open Information Security Foundation contribution agreement at https://suricata.io/about/contribution-agreement/
• [✓] I have updated the user guide (in doc/userguide/) to reflect the changes made (if applicable)

Link to redmine ticket:
https://redmine.openinfosecfoundation.org/issues/6222

Describe changes:
Set the L4 proto before decoding GRE in ipv6 decoding in case there is a GRE header decoding error.

OISF/suricata-verify/pull/1320

SV_BRANCH=pr/1320

[codecov]

Codecov Report

Merging #9262 (51ec90b) into master (9a33c53) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9262      +/-   ##
==========================================
- Coverage   82.42%   82.41%   -0.01%     
==========================================
  Files         968      968              
  Lines      273952   273953       +1     
==========================================
- Hits       225807   225787      -20     
- Misses      48145    48166      +21     

Flag	Coverage Δ
fuzzcorpus	64.69% <100.00%> (-0.01%)	⬇️
suricata-verify	60.79% <100.00%> (-0.01%)	⬇️
unittests	62.93% <0.00%> (-0.01%)	⬇️

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[victorjulien]

Merged in #9267, thanks!