UBSAN in test_tss

Output: https://travis-ci.org/jeking3/thread/jobs/457490782#L1351
```
gcc.compile.c++ ../../bin.v2/libs/thread/test/test_xtime_lib.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/tss_null.o
gcc.link ../../bin.v2/libs/thread/test/test_tss.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_tss
testing.capture-output ../../bin.v2/libs/thread/test/test_tss.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_tss.run
====== BEGIN OUTPUT ======
../../libs/thread/src/pthread/thread.cpp:114:60: runtime error: member call on address 0x000001659b40 which does not point to an object of type 'tss_cleanup_function'
0x000001659b40: note: object is of type 'boost::thread_specific_ptr::delete_data'
00 00 00 00 40 df 46 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 31 00 00 00
^~~~~~~~~~~~~~~~~~~~~~~
vptr for 'boost::thread_specific_ptr::delete_data'
#0 0x7fe7adff437d in tls_destructor ../../libs/thread/src/pthread/thread.cpp:114
#1 0x7fe7adff487d in thread_proxy ../../libs/thread/src/pthread/thread.cpp:194
#2 0x7fe7ac1c2183 in start_thread (/lib/x86_64-linux-gnu/libpthread.so.0+0x8183)
#3 0x7fe7abeeeffc 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 $status -ne 0 ; then
echo Skipping test execution due to testing.execute=off
exit 0
fi
"../../bin.v2/libs/thread/test/test_tss.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_tss" > "../../bin.v2/libs/thread/test/test_tss.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_tss.output" 2>&1 < /dev/null
status=$?
echo >> "../../bin.v2/libs/thread/test/test_tss.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_tss.output"
echo EXIT STATUS: $status >> "../../bin.v2/libs/thread/test/test_tss.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_tss.output"
if test $status -eq 0 ; then
cp "../../bin.v2/libs/thread/test/test_tss.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_tss.output" "../../bin.v2/libs/thread/test/test_tss.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_tss.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_tss.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_tss.output"
echo ====== END OUTPUT ======
fi
exit $status
...failed testing.capture-output ../../bin.v2/libs/thread/test/test_tss.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_tss.run...
```

Discovered in Travis CI; repeatable 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@bdfbeceb79ff:/boost/libs/thread/test$ ubsan test_tss
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_tss
Performing configuration checks

- default address-model : 64-bit (cached)
- default architecture : x86 (cached)
- symlinks supported : yes (cached)
- BOOST_COMP_GNUC >= 4.3.0 : yes (cached)
- lockfree boost::atomic_flag : yes (cached)
...patience...
...patience...
...found 3530 targets...
...updating 10 targets...
gcc.compile.c++ ../../../bin.v2/libs/thread/test/test_tss.test/gcc-8/debug/threadapi-pthread/threading-multi/visibility-hidden/test_tss.o
gcc.compile.c++ ../../../bin.v2/libs/thread/test/test_tss.test/gcc-8/debug/threadapi-pthread/threading-multi/visibility-hidden/winrt_init.o
gcc.link ../../../bin.v2/libs/thread/test/test_tss.test/gcc-8/debug/threadapi-pthread/threading-multi/visibility-hidden/test_tss
testing.capture-output ../../../bin.v2/libs/thread/test/test_tss.test/gcc-8/debug/threadapi-pthread/threading-multi/visibility-hidden/test_tss.run
====== BEGIN OUTPUT ======
../../../libs/thread/src/pthread/thread.cpp:114:60: runtime error: member call on address 0x55ab4a8657e0 which does not point to an object of type 'tss_cleanup_function'
0x55ab4a8657e0: note: object is of type 'boost::thread_specific_ptr::delete_data'
00 00 00 00 e0 2c ba 49 ab 55 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 31 00 00 00
^~~~~~~~~~~~~~~~~~~~~~~
vptr for 'boost::thread_specific_ptr::delete_data'

EXIT STATUS: 1
====== END OUTPUT ======
```

test_11053 looks similar: https://travis-ci.org/jeking3/thread/jobs/457490782#L6358
```
gcc.compile.c++ ../../bin.v2/libs/thread/test/test_11053.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_11053.o
gcc.link ../../bin.v2/libs/thread/test/test_11053.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_11053
testing.capture-output ../../bin.v2/libs/thread/test/test_11053.test/gcc-8/debug/cxxstd-03-iso/threadapi-pthread/threading-multi/visibility-hidden/test_11053.run
====== BEGIN OUTPUT ======
../../libs/thread/src/pthread/thread.cpp:114:60: runtime error: member call on address 0x000001feedc0 which does not point to an object of type 'tss_cleanup_function'
0x000001feedc0: note: object is of type 'boost::thread_specific_ptr::delete_data'
00 00 00 00 90 d2 42 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 31 00 00 00
^~~~~~~~~~~~~~~~~~~~~~~
vptr for 'boost::thread_specific_ptr::delete_data'
#0 0x7f0786cf937d in tls_destructor ../../libs/thread/src/pthread/thread.cpp:114
#1 0x7f0786cf987d in thread_proxy ../../libs/thread/src/pthread/thread.cpp:194
#2 0x7f078570e183 in start_thread (/lib/x86_64-linux-gnu/libpthread.so.0+0x8183)
#3 0x7f078543affc in clone (/lib/x86_64-linux-gnu/libc.so.6+0xfdffc)
EXIT STATUS: 1
====== END OUTPUT ======
```