(related: tdf#80715) sw: fix Ring::MoveTo() not doing anything...

... if the parameter is currently in the same list.

The "boost::intrusive::circular_list_algorithms::transfer" has a
precondition that the 2 parameters must not be in the same list.

This causes an infinite loop in SwFindParaText::Find(), which is hiding
the infinite loop that i'm trying to debug...

While at it, remove some unnecessary complexity.

Change-Id: Ib41f52c6d5c44ecc358c6170ee1e6e98729e1302

sw/inc/ring.hxx

@@ -135,16 +135,12 @@ namespace sw
     inline void Ring<value_type>::MoveTo(value_type* pDestRing)
     {
         value_type* pThis = static_cast< value_type* >(this);
+        algo::unlink(pThis);
         // insert into "new"
-        if( pDestRing )
+        if (pDestRing)
         {
-            if(algo::unique(pThis))
-                algo::link_before(pDestRing, pThis);
-            else
-                algo::transfer(pDestRing, pThis);
+            algo::link_before(pDestRing, pThis);
         }
-        else
-            algo::unlink(pThis);
     }
 
     /**

sw/qa/core/uwriter.cxx

@@ -1372,6 +1372,15 @@ void SwDocTest::testIntrusiveRing()
         const TestRing* pRing = &r;
         CPPUNIT_ASSERT(pRing);
     }
+    TestRing foo, bar;
+    foo.MoveTo(&bar);
+    CPPUNIT_ASSERT_EQUAL(&foo, bar.GetNext());
+    CPPUNIT_ASSERT_EQUAL(&foo, bar.GetPrev());
+    CPPUNIT_ASSERT_EQUAL(&bar, foo.GetNext());
+    CPPUNIT_ASSERT_EQUAL(&bar, foo.GetPrev());
+    foo.MoveTo(&foo);
+    CPPUNIT_ASSERT_EQUAL(&bar, bar.GetNext());
+    CPPUNIT_ASSERT_EQUAL(&bar, bar.GetPrev());
 }
 
 void SwDocTest::setUp()