conditionvar_timeout intermittently fails on maint-0.3.3

util/thread/conditionvar_timeout failed in the last maint-0.3.3 build: https://travis-ci.org/torproject/tor/builds/411701508

With the following message:

util/thread/conditionvar_timeout: [forking] 
  FAIL src/test/test_threads.c:285: assert(ti->n_timeouts OP_EQ 2): 1 vs 2Aug 03 13:03:59.190 [err] Error 16 destroying a mutex.
Aug 03 13:03:59.190 [err] tor_assertion_failed_(): Bug: src/common/compat_pthreads.c:172: tor_mutex_uninit: Assertion 0 failed; aborting. (on Tor 0.3.3.8-dev 2a6c1585b0f13e03)
Aug 03 13:03:59.191 [err] Bug: Assertion 0 failed in tor_mutex_uninit at src/common/compat_pthreads.c:172. Stack trace: (on Tor 0.3.3.8-dev 2a6c1585b0f13e03)
Aug 03 13:03:59.191 [err] Bug:     ./src/test/test(log_backtrace+0x46) [0x55cd7c253ec6] (on Tor 0.3.3.8-dev 2a6c1585b0f13e03)
Aug 03 13:03:59.191 [err] Bug:     ./src/test/test(tor_assertion_failed_+0xe4) [0x55cd7c294434] (on Tor 0.3.3.8-dev 2a6c1585b0f13e03)
Aug 03 13:03:59.191 [err] Bug:     ./src/test/test(tor_mutex_uninit+0xa6) [0x55cd7c29d736] (on Tor 0.3.3.8-dev 2a6c1585b0f13e03)
Aug 03 13:03:59.191 [err] Bug:     ./src/test/test(tor_mutex_free_+0x25) [0x55cd7c262dc5] (on Tor 0.3.3.8-dev 2a6c1585b0f13e03)
Aug 03 13:03:59.191 [err] Bug:     ./src/test/test(+0x54508a) [0x55cd7bed908a] (on Tor 0.3.3.8-dev 2a6c1585b0f13e03)
Aug 03 13:03:59.191 [err] Bug:     ./src/test/test(+0x5c840d) [0x55cd7bf5c40d] (on Tor 0.3.3.8-dev 2a6c1585b0f13e03)
Aug 03 13:03:59.191 [err] Bug:     ./src/test/test(testcase_run_one+0x310) [0x55cd7bf5c7d0] (on Tor 0.3.3.8-dev 2a6c1585b0f13e03)
Aug 03 13:03:59.191 [err] Bug:     ./src/test/test(tinytest_main+0x1eb) [0x55cd7bf5d58b] (on Tor 0.3.3.8-dev 2a6c1585b0f13e03)
Aug 03 13:03:59.191 [err] Bug:     ./src/test/test(main+0x4e9) [0x55cd7bbb61b9] (on Tor 0.3.3.8-dev 2a6c1585b0f13e03)
Aug 03 13:03:59.191 [err] Bug:     /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf5) [0x2b2813951f45] (on Tor 0.3.3.8-dev 2a6c1585b0f13e03)
Aug 03 13:03:59.191 [err] Bug:     ./src/test/test(+0x224fdb) [0x55cd7bbb8fdb] (on Tor 0.3.3.8-dev 2a6c1585b0f13e03)
[Lost connection!] 
  [conditionvar_timeout FAILED]