Fix fibers with tracer disabled

[bwoebi]

Description

Fibers were crashing when using ddtrace.disable=1.

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-03-19 12:21:00

Comparing candidate commit 87effa0 in PR branch bob/fix-fibers with baseline commit 91227e5 in branch master.

Found 0 performance improvements and 3 performance regressions! Performance is the same for 179 metrics, 0 unstable metrics.

scenario:PDOBench/benchPDOBaseline

• 🟥 execution_time [+15.779µs; +16.391µs] or [+9.108%; +9.461%]

scenario:PDOBench/benchPDOOverhead

• 🟥 execution_time [+14.299µs; +16.086µs] or [+5.310%; +5.974%]

scenario:PDOBench/benchPDOOverheadWithDBM

• 🟥 execution_time [+15.079µs; +17.317µs] or [+5.178%; +5.946%]

[codecov-commenter]

Codecov Report

Merging #2576 (87effa0) into master (91227e5) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #2576   +/-   ##
=========================================
  Coverage     75.77%   75.77%           
  Complexity     2563     2563           
=========================================
  Files           241      241           
  Lines         27024    27024           
  Branches        976      976           
=========================================
  Hits          20478    20478           
  Misses         6026     6026           
  Partials        520      520           

Flag	Coverage Δ
appsec-extension	69.13% <ø> (ø)
tracer-extension	78.70% <100.00%> (ø)
tracer-php	74.71% <ø> (ø)

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

Files	Coverage Δ
ext/ddtrace.c	73.13% <100.00%> (ø)

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