UBSAN in test_thread_exit

Output: https://travis-ci.org/jeking3/thread/jobs/457490782#L1226
```
gcc.compile.c++ ../../bin.v2/libs/thread/test/test_thread_exit_lib.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/winrt_init.o
gcc.compile.c++ ../../bin.v2/libs/thread/test/test_thread_exit_lib.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/tss_null.o
gcc.compile.c++ ../../bin.v2/libs/thread/test/test_thread_exit.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_thread_exit.o
gcc.link ../../bin.v2/libs/thread/test/test_thread_exit.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_thread_exit
testing.capture-output ../../bin.v2/libs/thread/test/test_thread_exit.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_thread_exit.run
====== BEGIN OUTPUT ======
../../libs/thread/src/pthread/thread.cpp:103:58: runtime error: member call on address 0x7f2ce80008c0 which does not point to an object of type 'thread_exit_function_base'
0x7f2ce80008c0: note: object is of type 'boost::detail::thread_exit_function'
00 00 00 00 d0 f2 42 00 00 00 00 00 94 55 40 00 00 00 00 00 00 00 00 00 00 00 00 00 25 00 00 00
^~~~~~~~~~~~~~~~~~~~~~~
vptr for 'boost::detail::thread_exit_function'
#0 0x7f2cf1084d90 in tls_destructor ../../libs/thread/src/pthread/thread.cpp:103
#1 0x7f2cf108587d in thread_proxy ../../libs/thread/src/pthread/thread.cpp:194
#2 0x7f2cef45b183 in start_thread (/lib/x86_64-linux-gnu/libpthread.so.0+0x8183)
#3 0x7f2cef187ffc in clone (/lib/x86_64-linux-gnu/libc.so.6+0xfdffc)
EXIT STATUS: 1
====== END OUTPUT ======
LD_LIBRARY_PATH="/home/travis/build/jeking3/boost-root/bin.v2/libs/chrono/build/gcc-8/debug/cxxstd-03-iso/threading-multi/visibility-hidden:/home/travis/build/jeking3/boost-root/bin.v2/libs/system/build/gcc-8/debug/cxxstd-03-iso/threading-multi/visibility-hidden:/home/travis/build/jeking3/boost-root/bin.v2/libs/test/build/gcc-8/debug/cxxstd-03-iso/threading-multi/visibility-hidden:/home/travis/build/jeking3/boost-root/bin.v2/libs/thread/build/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden:/home/travis/build/jeking3/boost-root/bin.v2/libs/timer/build/gcc-8/debug/cxxstd-03-iso/threading-multi/visibility-hidden:/usr/bin:/usr/lib:/usr/lib32:/usr/lib64:$LD_LIBRARY_PATH"
export LD_LIBRARY_PATH
status=0
if test $status -ne 0 ; then
echo Skipping test execution due to testing.execute=off
exit 0
fi
"../../bin.v2/libs/thread/test/test_thread_exit.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_thread_exit" > "../../bin.v2/libs/thread/test/test_thread_exit.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_thread_exit.output" 2>&1 < /dev/null
status=$?
echo >> "../../bin.v2/libs/thread/test/test_thread_exit.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_thread_exit.output"
echo EXIT STATUS: $status >> "../../bin.v2/libs/thread/test/test_thread_exit.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_thread_exit.output"
if test $status -eq 0 ; then
cp "../../bin.v2/libs/thread/test/test_thread_exit.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_thread_exit.output" "../../bin.v2/libs/thread/test/test_thread_exit.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_thread_exit.run"
fi
verbose=0
if test $status -ne 0 ; then
verbose=1
fi
if test $verbose -eq 1 ; then
echo ====== BEGIN OUTPUT ======
cat "../../bin.v2/libs/thread/test/test_thread_exit.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_thread_exit.output"
echo ====== END OUTPUT ======
fi
exit $status
...failed testing.capture-output ../../bin.v2/libs/thread/test/test_thread_exit.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_thread_exit.run...
```

Repeatable with the following command using the [Boost Docker Development Environment](https://github.com/jeking3/bdde):
```
jking@ubuntu:~/boost/libs/thread/test$ bdde
docker run -v /home/jking/boost:/boost:rw -v /home/jking/bdde:/bdde:ro --workdir /boost/libs/thread/test -it jeking3/bdde:linux /bin/bash

boost@907e1fe89581:/boost/libs/thread/test$ ubsan test_thread_exit
UBSAN_OPTIONS=print_stacktrace=1 b2 define=BOOST_NO_STRESS_TEST=1 cxxflags=-fno-omit-frame-pointer cxxflags=-fsanitize=undefined cxxflags=-fno-sanitize-recover=undefined linkflags=-fsanitize=undefined linkflags=-fno-sanitize-recover=undefined linkflags=-fuse-ld=gold variant=debug toolset=gcc-8 test_thread_exit
```