[loader] Fix crash when the loader is loaded multiple times

[iamluc]

Description

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.74%. Comparing base (02fe693) to head (6b2a17c).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #2954   +/-   ##
=========================================
  Coverage     70.74%   70.74%           
  Complexity     2741     2741           
=========================================
  Files           110      110           
  Lines         10833    10833           
=========================================
  Hits           7664     7664           
  Misses         3169     3169           

Flag	Coverage Δ
tracer-php	70.74% <ø> (ø)

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[bwoebi]

Looks good :-)