Add minimal test for UBSan failures

Reproduced with clang 5.0 and 6.0 on Linux using Boost super-project at
master as well as Function at develop:

cd libs/function/test/ubsan_undefined_bug
b2 toolset=clang variant=ubsan_undefined

test/ubsan_undefined_bug/test.cpp

@@ -0,0 +1,11 @@
+#define BOOST_TEST_MAIN
+#include <boost/test/unit_test.hpp>
+
+BOOST_AUTO_TEST_SUITE( ubsan_tests )
+
+BOOST_AUTO_TEST_CASE( test1 )
+{
+    BOOST_CHECK(true);
+}
+
+BOOST_AUTO_TEST_SUITE_END()