fix: defensive reads for all thread locals

[zhongzc]

fix #160

[coveralls]

Pull Request Test Coverage Report for Build 5916385128

• 27 of 37 (72.97%) changed or added relevant lines in 9 files are covered.
• 3 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.3%) to 82.849%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
minitrace/src/collector/id.rs	7	8	87.5%
minitrace/src/local/local_collector.rs	2	3	66.67%
minitrace/src/local/local_span.rs	2	3	66.67%
minitrace/src/local/span_queue.rs	1	2	50.0%
minitrace/src/event.rs	0	2	0.0%
minitrace/src/span.rs	5	7	71.43%
minitrace/src/util/mod.rs	4	6	66.67%

Files with Coverage Reduction	New Missed Lines	%
minitrace/src/local/local_collector.rs	1	94.32%
minitrace/src/local/local_span.rs	1	92.19%
minitrace/src/span.rs	1	97.44%

Totals
Change from base Build 5689564312:	-0.3%
Covered Lines:	1652
Relevant Lines:	1994

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💛 - Coveralls

[drmingdrmer]

Thank you for this quick fix! We'll proceed to examine it out in our environment as soon as it merged!