Fix emplace_back return type also in compilers without variadic templates. Update changelog.

include/boost/container/vector.hpp

@@ -1794,10 +1794,11 @@ class vector
    BOOST_CONTAINER_FORCEINLINE reference emplace_back(BOOST_MOVE_UREF##N)\
    {\
       if (BOOST_LIKELY(this->room_enough())){\
+         T* const p = this->priv_raw_end();\
          allocator_traits_type::construct (this->m_holder.alloc()\
             , this->priv_raw_end() BOOST_MOVE_I##N BOOST_MOVE_FWD##N);\
          ++this->m_holder.m_size;\
-         return *this->priv_raw_end();\
+         return *p;\
       }\
       else{\
          typedef container_detail::insert_emplace_proxy_arg##N<Allocator, T* BOOST_MOVE_I##N BOOST_MOVE_TARG##N> type;\