sw: fix odd condition in SwpHints::MergePortions()

All of the RES_CHRATR_* items are poolable, hence we have
(pItem1 != pItem2) == (pItem1->Which() != pItem2->Which() || *pItem1 != *pItem2)

Move the redundant check to an assert() so we notice in case
somebody adds a new non-poolable RES_CHRATR.

Thanks to ccsheller for pointing me to this condition.

Change-Id: I9e0634946b8bede3f483bb8997f69de05beae64c

sw/source/core/txtnode/thints.cxx

@@ -2782,10 +2782,9 @@ bool SwpHints::MergePortions( SwTextNode& rNode )
                                  pItem1 = iter1.NextItem(),
                                  pItem2 = iter2.NextItem())
                             {
-                                if (pItem1 != pItem2 ||
-                                    pItem1->Which() != pItem2->Which() ||
-                                    *pItem1 != *pItem2)
+                                if (pItem1 != pItem2) // all are poolable
                                 {
+                                    assert(IsInvalidItem(pItem1) || IsInvalidItem(pItem2) || pItem1->Which() != pItem2->Which() || *pItem1 != *pItem2);
                                     eMerge = DIFFER;
                                     break;
                                 }