Work around odd -Werror=strict-overflow

"sc/source/filter/oox/pivotcachebuffer.cxx:401:45: error: assuming signed
overflow does not occur when assuming that (X + c) < X is always false" at least
with "g++ (GCC) 5.1.1 20150618 (Red Hat 5.1.1-4)" and --disable-debug

Change-Id: I1d267b2947f04b84f73da2b1458d316397e2374e

sc/source/filter/oox/pivotcachebuffer.cxx

@@ -408,7 +408,7 @@ void PivotCacheItemList::importArray( SequenceInputStream& rStrm )
             case BIFF12_PCITEM_ARRAY_DATE:   createItem().readDate( rStrm );     break;
             default:
                 OSL_FAIL( "PivotCacheItemList::importArray - unknown data type" );
-                nIdx = nCount;
+                return;
         }
     }
 }