unit.lf failed in buildbot, sporadic

unit.lf                                  w1 [ fail ]

        Test ended at 2017-11-21 20:07:58

CURRENT_TEST: unit.lf

# N CPUs: 4, atomic ops: gcc-atomics-smp

1..7

ok 1 - my_atomic_initialize() returned 0

# Testing lf_pinbox (with my_thread_init) with 30 threads, 3000 iterations... 

ok 2 - tested lf_pinbox (with my_thread_init) in 0.0358399 secs (0)

# Testing lf_alloc (with my_thread_init) with 30 threads, 3000 iterations... 

Bail out! Signal 11 thrown

# 7 tests planned,  0 failed,  2 was last executed

mysqltest failed with unexpected return code 255

 - saving '/mnt/buildbot/build/mariadb-10.1.30/mysql-test/var/1/log/unit.lf/' to '/mnt/buildbot/build/mariadb-10.1.30/mysql-test/var/log/unit.lf/'