Fix parallel hash span creation#88
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What does this PR do?
Avoids dereferencing a missing traced child when creating a Hash span. When that happens, the span uses the parent start as the available bound.
Motivation
We saw repeated segfaults from sampled parallel hash plans. A parallel worker can have Hash instrumentation even when its child did not pass through the tracing wrapper in that worker.
Thanks for maintaining pg_tracing. It has been very useful for us.
Additional Notes
Adds a regression test for the parallel hash case. PG17 pgindent and all 27 regression tests pass.