Fix crash with double parent key in tracestate

[bwoebi]

Access of span_parent_key would be NULL on second 'p'.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.49%. Comparing base (735db41) to head (a688a78).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2837      +/-   ##
============================================
- Coverage     81.10%   78.49%   -2.62%     
  Complexity     2516     2516              
============================================
  Files           146      173      +27     
  Lines         14651    18680    +4029     
  Branches          0      975     +975     
============================================
+ Hits          11883    14662    +2779     
- Misses         2768     3481     +713     
- Partials          0      537     +537     

Flag	Coverage Δ
appsec-extension	69.12% <ø> (?)
tracer-extension	78.17% <ø> (ø)
tracer-php	82.27% <ø> (-0.06%)	⬇️

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see 28 files with indirect coverage changes

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