UBSAN identified issue in test

https://travis-ci.org/boostorg/iostreams/jobs/448302953#L1170
```
testing.capture-output ../../bin.v2/libs/iostreams/test/code_converter_test.test/gcc-7/debug/cxxstd-03-iso/visibility-hidden/code_converter_test.run
====== BEGIN OUTPUT ======
../../libs/iostreams/test/code_converter_test.cpp:205:5: runtime error: member call on address 0x7ffd672455d0 which does not point to an object of type '__codecvt_abstract_base'
0x7ffd672455d0: note: object is of type 'utf8_codecvt_facet'
00 00 00 00 40 9a 6a 00 00 00 00 00 00 00 00 00 00 00 00 00 00 84 ba 66 bb 7f 00 00 c3 3c dd 66
^~~~~~~~~~~~~~~~~~~~~~~
vptr for 'utf8_codecvt_facet'
#0 0x41cc18 in bool codecvt_test1 >() (/home/travis/build/boostorg/boost-root/bin.v2/libs/iostreams/test/code_converter_test.test/gcc-7/debug/cxxstd-03-iso/visibility-hidden/code_converter_test+0x41cc18)
#1 0x419b54 in bool codecvt_test >() (/home/travis/build/boostorg/boost-root/bin.v2/libs/iostreams/test/code_converter_test.test/gcc-7/debug/cxxstd-03-iso/visibility-hidden/code_converter_test+0x419b54)
#2 0x40f02a in code_converter_test() ../../libs/iostreams/test/code_converter_test.cpp:254
#3 0x43646e in boost::detail::function::void_function_invoker0::invoke(boost::detail::function::function_buffer&) (/home/travis/build/boostorg/boost-root/bin.v2/libs/iostreams/test/code_converter_test.test/gcc-7/debug/cxxstd-03-iso/visibility-hidden/code_converter_test+0x43646e)
#4 0x5d1d68 in boost::function0::operator()() const ../../boost/function/function_template.hpp:829
#5 0x61f1ba in boost::detail::forward::operator()() ../../boost/test/impl/execution_monitor.ipp:1312
#6 0x624929 in boost::detail::function::function_obj_invoker0::invoke(boost::detail::function::function_buffer&) ../../boost/function/function_template.hpp:172
#7 0x622c7e in boost::function0::operator()() const ../../boost/function/function_template.hpp:830
#8 0x620870 in int boost::detail::do_invoke, boost::function >(boost::shared_ptr const&, boost::function const&) ../../boost/test/impl/execution_monitor.ipp:286
#9 0x618c7b in boost::execution_monitor::catch_signals(boost::function const&) ../../boost/test/impl/execution_monitor.ipp:875
#10 0x618f6c in boost::execution_monitor::execute(boost::function const&) ../../boost/test/impl/execution_monitor.ipp:1214
#11 0x61afd0 in boost::execution_monitor::vexecute(boost::function const&) ../../boost/test/impl/execution_monitor.ipp:1321
#12 0x60da09 in boost::unit_test::unit_test_monitor_t::execute_and_translate(boost::function const&, unsigned int) ../../boost/test/impl/unit_test_monitor.ipp:49
#13 0x5cf928 in boost::unit_test::framework::state::execute_test_tree(unsigned long, unsigned int, boost::unit_test::framework::state::random_generator_helper const*) ../../boost/test/impl/framework.ipp:789
#14 0x5cefc4 in boost::unit_test::framework::state::execute_test_tree(unsigned long, unsigned int, boost::unit_test::framework::state::random_generator_helper const*) ../../boost/test/impl/framework.ipp:737
#15 0x5cefc4 in boost::unit_test::framework::state::execute_test_tree(unsigned long, unsigned int, boost::unit_test::framework::state::random_generator_helper const*) ../../boost/test/impl/framework.ipp:737
#16 0x5bf706 in boost::unit_test::framework::run(unsigned long, bool) ../../boost/test/impl/framework.ipp:1631
#17 0x4fbe3d in boost::unit_test::unit_test_main(boost::unit_test::test_suite* (*)(int, char**), int, char**) ../../boost/test/impl/unit_test_main.ipp:247
#18 0x4fcba0 in main ../../boost/test/impl/unit_test_main.ipp:303
#19 0x7fbb66809f44 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21f44)
#20 0x40c408 (/home/travis/build/boostorg/boost-root/bin.v2/libs/iostreams/test/code_converter_test.test/gcc-7/debug/cxxstd-03-iso/visibility-hidden/code_converter_test+0x40c408)
EXIT STATUS: 1
====== END OUTPUT ======
LD_LIBRARY_PATH="/usr/bin:/usr/lib:/usr/lib32:/usr/lib64:$LD_LIBRARY_PATH"
export LD_LIBRARY_PATH
status=0
if test $status -ne 0 ; then
echo Skipping test execution due to testing.execute=off
exit 0
fi
"../../bin.v2/libs/iostreams/test/code_converter_test.test/gcc-7/debug/cxxstd-03-iso/visibility-hidden/code_converter_test" > "../../bin.v2/libs/iostreams/test/code_converter_test.test/gcc-7/debug/cxxstd-03-iso/visibility-hidden/code_converter_test.output" 2>&1 < /dev/null
status=$?
echo >> "../../bin.v2/libs/iostreams/test/code_converter_test.test/gcc-7/debug/cxxstd-03-iso/visibility-hidden/code_converter_test.output"
echo EXIT STATUS: $status >> "../../bin.v2/libs/iostreams/test/code_converter_test.test/gcc-7/debug/cxxstd-03-iso/visibility-hidden/code_converter_test.output"
if test $status -eq 0 ; then
cp "../../bin.v2/libs/iostreams/test/code_converter_test.test/gcc-7/debug/cxxstd-03-iso/visibility-hidden/code_converter_test.output" "../../bin.v2/libs/iostreams/test/code_converter_test.test/gcc-7/debug/cxxstd-03-iso/visibility-hidden/code_converter_test.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/iostreams/test/code_converter_test.test/gcc-7/debug/cxxstd-03-iso/visibility-hidden/code_converter_test.output"
echo ====== END OUTPUT ======
fi
exit $status
...failed testing.capture-output ../../bin.v2/libs/iostreams/test/code_converter_test.test/gcc-7/debug/cxxstd-03-iso/visibility-hidden/code_converter_test.run...
```
Marking the test as a known failure.

Reproduce the UBSAN failure in libs/iostreams/test/code_converter_test.cpp, especially line 205, using the GCC 7 C++03 configuration shown in the report. Read the codecvt test path from codecvt_test1 through code_converter_test at line 254 and determine why the member call targets the wrong object type. Done means the test runs without this UBSAN error and no longer needs to be marked as a known failure.

Written by the indexing model from the issue text.