RandomAccessIterator + 0

Previously the iterators of boost::container::deque would assert when
zero was added to them in at least the following situations:

- The iterator was obtained by a call to boost::container::deque::begin
  and boost::container::deque::empty returns true
- The iterator was default constructed

This is inconsistent with the way in which the iterators of boost::
container::deque have behaved historically and is also inconsistent
with an understanding of iterators developed by analogy with pointers:

- Adding zero to a null pointer is valid despite the fact the null
  pointer cannot be dereferenced
- Adding zero to a pointer that is one past the end yields a pointer
  which is still one past the end and thus which has well defined
  semantics

Fixed this issue and codified the expected behavior in unit tests.

include/boost/container/deque.hpp

@@ -234,6 +234,8 @@ class deque_iterator
 
    deque_iterator& operator+=(difference_type n) BOOST_NOEXCEPT_OR_NOTHROW
    {
+      if (!n)
+         return *this;
       BOOST_ASSERT(!!m_cur);
       difference_type offset = n + (this->m_cur - this->m_first);
       const difference_type block_size = this->m_last - this->m_first;