Possible race condition in thread get_id()

[brycelelbach]

To reproduce, run the thread unit test.

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Command: ./bin/thread_test -t4 -l1 -0
Result: Failed
Exit code: 1
Timed out: False
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/opt/buildbot/slave/gcc_cxx03_release_commit_check/build/tests/unit/threads/thread.cpp(288): test 't1.get_id() == id2' failed in function 'void test_swap()'
/opt/buildbot/slave/gcc_cxx03_release_commit_check/build/tests/unit/threads/thread.cpp(293): test 't2.get_id() == id2' failed in function 'void test_swap()'
0 sanity checks and 2 tests failed.
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[sithhell]

I think this is due to the thread of execution finished before get_id is called.