avoid another hang with certain .docs

(cherry picked from commit 0592da19)

Change-Id: If16e90c5ba1a43ceb9702e752835928da7b3ef32
Reviewed-on: https://gerrit.libreoffice.org/17657Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>

sw/source/filter/ww8/ww8par.cxx

@@ -2286,7 +2286,7 @@ void SwWW8ImplReader::Read_HdFtText(WW8_CP nStart, WW8_CP nLen, SwFrameFormat* p
 bool SwWW8ImplReader::isValid_HdFt_CP(WW8_CP nHeaderCP) const
 {
     // Each CP of Plcfhdd MUST be less than FibRgLw97.ccpHdd
-    return (nHeaderCP < m_pWwFib->ccpHdr);
+    return (nHeaderCP < m_pWwFib->ccpHdr && nHeaderCP >= 0);
 }
 
 bool SwWW8ImplReader::HasOwnHeaderFooter(sal_uInt8 nWhichItems, sal_uInt8 grpfIhdt,
@@ -2304,7 +2304,7 @@ bool SwWW8ImplReader::HasOwnHeaderFooter(sal_uInt8 nWhichItems, sal_uInt8 grpfIh
             {
                 bool bOk = true;
                 if( m_bVer67 )
-                    bOk = ( m_pHdFt->GetTextPos(grpfIhdt, nI, start, nLen ) && nLen >= 2 );
+                    bOk = ( m_pHdFt->GetTextPos(grpfIhdt, nI, start, nLen ) && start >= 0 && nLen >= 2 );
                 else
                 {
                     m_pHdFt->GetTextPosExact( static_cast< short >(nNumber + (nSect+1)*6), start, nLen);