ofz#6449 Integer-overflow

Change-Id: Ia144192a1374615c50fb3574f7a229926e43d7cb
Reviewed-on: https://gerrit.libreoffice.org/49951Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

filter/source/graphicfilter/itiff/itiff.cxx

@@ -1363,7 +1363,7 @@ bool TIFFReader::ReadTIFF(SvStream & rTIFF, Graphic & rGraphic )
             }
             if ( !nBitsPerSample || ( nBitsPerSample > 32 ) )
                 bStatus = false;
-            if (nImageWidth < 0 || nImageLength < 0)
+            if (nImageWidth <= 0 || nImageLength <= 0)
                 bStatus = false;
             if ( bStatus )
             {