Ensure DDTRACE_G(active_stack) is always NULL when the root span is being pushed

[bwoebi]

I have zero idea how this happened, given that I'm unable to reproduce it, but a core dump indicated that the memory it pointed to was from the previous request. While this is not a very proper fix (as this should not be possible in the first place), it mitigates the effects.

So, better a mitigation than a crash... :-)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.96%. Comparing base (c5f2dcb) to head (4157bd2). Report is 1 commits behind head on master.

Additional details and impacted files

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Benchmarks

Benchmark execution time: 2024-06-10 11:28:42

Comparing candidate commit 4157bd27b36b8d8284017781bd00ee988e63ffa4 in PR branch bob/safety-stack with baseline commit c5f2dcb3faec360227837310221040ce6208e082 in branch master.

Found 3 performance improvements and 1 performance regressions! Performance is the same for 174 metrics, 0 unstable metrics.

scenario:LaravelBench/benchLaravelOverhead

• 🟥 execution_time [+58.800µs; +201.920µs] or [+2.151%; +7.387%]

scenario:PDOBench/benchPDOBaseline

• 🟩 execution_time [-17.142µs; -14.850µs] or [-9.033%; -7.825%]

scenario:PDOBench/benchPDOOverhead

• 🟩 execution_time [-18.637µs; -16.546µs] or [-6.388%; -5.671%]

scenario:PDOBench/benchPDOOverheadWithDBM

• 🟩 execution_time [-17.671µs; -15.962µs] or [-5.585%; -5.045%]