when_all() on a deferred future crashes

Reproduce the deferred-future example and inspect boost/thread/future.hpp at run_if_is_deferred, accumulate_run_if_is_deferred, and the when_all tuple shared state. Trace why when_all reaches the assertion, then add a regression test covering the shown call; done means the example no longer crashes and the test passes.

Trying to run

  int q = 0;
  boost::promise<int> x;
  boost::future<int> fx = x.get_future();
  fx.then(boost::launch::deferred, [&q](boost::future<int>&&) { q = 1; });
  future<std::tuple<boost::future<int>>>&& _ = boost::when_all(std::move(fx)); // CRASH!

Yields a crash:

#0  0x00007ffff4ae1387 in raise () from /lib64/libc.so.6
#1  0x00007ffff4ae2a78 in abort () from /lib64/libc.so.6
#2  0x00007ffff4ada1a6 in __assert_fail_base () from /lib64/libc.so.6
#3  0x00007ffff4ada252 in __assert_fail () from /lib64/libc.so.6
#4  0x000000000307ceaa in boost::shared_ptr<boost::detail::shared_state<int> >::operator-> (this=0xec0eb60)
    at boost/smart_ptr/shared_ptr.hpp:734
#5  0x0000000003094d85 in boost::future<int>::run_if_is_deferred (
        this=0xec0eb60)
    at boost/thread/future.hpp:1785
#6  0x000000000308f5f7 in boost::detail::accumulate_run_if_is_deferred<std::tuple<boost::future<int> >, 1ul>::operator() (this=0x7fffffffd77f, 
        t=std::tuple containing)
    at boost/thread/future.hpp:5685
#7  0x0000000003086eb1 in boost::detail::future_when_all_tuple_shared_state<std::tuple<boost::future<int> >, boost::future<int>>::run_deferred() (
        this=0xec0ea40)
    at boost/thread/future.hpp:5726
#8  0x000000000307d24c in boost::detail::future_when_all_tuple_shared_state<std:
    :tuple<boost::future<int> >, boost::future<int>>::init() (this=0xec0ea40)
        at boost/thread/future.hpp:5729
#9  0x0000000003073cfd in boost::when_all<boost::future<int>>(boost::future<int>&&) (
            f=<unknown type in unittests, CU 0xa5fc9, DIE 0x19fe8a>)
    at boost/thread/future.hpp:5856

Which is maybe related to #112 asking for when_all to be rewritten anyway.