fix heap-use-after-free condition when cancelling spawned task, closes …

…boostorg#194

The variable "recs" was allocated on a piece of memory whose lifetime
was managed at the level of the coroutine (inside its frame). This led
to access to freed memory if that coroutine (its frame) was deleted

include/boost/cobalt/detail/spawn.hpp

@@ -11,8 +11,6 @@
 #include <boost/cobalt/task.hpp>
 #include <boost/asio/dispatch.hpp>
 
-#include <boost/smart_ptr/allocate_unique.hpp>
-
 namespace boost::cobalt
 {
 template<typename T>
@@ -46,7 +44,7 @@ struct async_initiate_spawn
 #else
     auto alloc = asio::get_associated_allocator(h);
 #endif
-    auto recs = std::allocate_shared<detail::task_receiver<T>>(alloc, std::move(rec));
+    auto recs = std::make_shared<detail::task_receiver<T>>(std::move(rec));
 
     auto sl = asio::get_associated_cancellation_slot(h);
     if (sl.is_connected())
@@ -108,7 +106,7 @@ struct async_initiate_spawn
 #else
     auto alloc = asio::get_associated_allocator(h);
 #endif
-    auto recs = std::allocate_shared<detail::task_receiver<void>>(alloc, std::move(a.receiver_));
+    auto recs = std::make_shared<detail::task_receiver<void>>(std::move(a.receiver_));
 
     if (recs->done)
       return asio::dispatch(asio::get_associated_immediate_executor(h, exec),