Write 0 of type 'unsigned char'

regression introduced with ac2d415a
"loplugin:oncevar in sc"

Change-Id: I1073e7e8b67ddf1dd633f004322c6bc46b9a8052

sc/source/filter/xcl97/xcl97rec.cxx

@@ -1587,7 +1587,7 @@ void XclExpSheetProtectOptions::WriteBody( XclExpStream& rStrm )
     rStrm << nBytes;
 
     for (int i = 0; i < 9; ++i)
-        rStrm << sal_Int32(0);
+        rStrm << static_cast<unsigned char>(0);
 
     nBytes = 0x0200;
     rStrm << nBytes;