Default case handling in exception_ptr missing clue for coverity to treat switch fall-through as intentional

[James E. King, III <jim.king@…>]

In exception_ptr.hpp lines 322 to 325:

                default:
                    BOOST_ASSERT(0);
                case exception_detail::clone_current_exception_result::
                not_supported:

Coverity complains about the fall-through. Changing it to the following will resolve this and make intentions clearer:

                default:
                    BOOST_ASSERT(0);
                    // fall through
                case exception_detail::clone_current_exception_result::
                not_supported:

This is present in the development trunk and goes back to at least 1.54.