editeng: avoid infinite loop in SvxBoxItem::Create

Apparently a sw unit test randomly read a partially written autotbl.fmt
and the stream has m_isEof = true.

Change-Id: Iacfbc42e83be6320d4aa695a5df4264f1db0b021

editeng/source/items/frmitems.cxx

@@ -2038,7 +2038,7 @@ SfxPoolItem* SvxBoxItem::Create( SvStream& rStrm, sal_uInt16 nIVersion ) const
                            SvxBoxItemLine::RIGHT, SvxBoxItemLine::BOTTOM };
 
     sal_Int8 cLine;
-    while( true )
+    while (rStrm.good())
     {
         rStrm.ReadSChar( cLine );