sw: RingContainer::merge(): assert that the Rings aren't already linked

The function would un-merge them in that case, which is just a little
bit surprising.

Change-Id: Ife1d572635b812d3ff5b9f93c1ddf1954e12aca5

sw/inc/ring.hxx

@@ -185,6 +185,10 @@ namespace sw
              */
             void merge( RingContainer< value_type > aDestRing )
             {
+                // first check that we aren't merged already, swapping would
+                // actually un-merge in this case!
+                assert(m_pStart->pPrev != aDestRing.m_pStart);
+                assert(m_pStart != aDestRing.m_pStart->pPrev);
                 std::swap(*(&m_pStart->pPrev->pNext), *(&aDestRing.m_pStart->pPrev->pNext));
                 std::swap(*(&m_pStart->pPrev), *(&aDestRing.m_pStart->pPrev));
             }