message: elle.reactor: introduce the base pointer
diff: @@ -120,8 +120,7 @@ namespace elle
  {
  #ifdef VALGRIND
  this->_valgrind_stack =
  - VALGRIND_STACK_REGISTER(
  - reinterpret_cast<char*>(this->_stack_pointer) - this->_stack_size,
  + VALGRIND_STACK_REGISTER(this->_base_pointer(),
  this->_stack_pointer);
  #endif
  }
  @@ -152,6 +151,13 @@ namespace elle
  ELLE_ASSERT(this->_context);
  }
  + /// The bottom of the stack.
  + void* _base_pointer() const
  + {
  + return static_cast<char*>(this->_stack_pointer) - this->_stack_size;
  + }
  +
  + /// Pass the execution from @from to @to.
  void _jump(Thread* from, Thread* to)
  {
  jump_fcontext(&from->_context, to->_context,
  