tdf#107709 filter: MSO2003XML import: fix invalid OLE lengths

The oleLength was -28160 for the bugdoc, so i guess the shifting of
signed chars there is perhaps not ideal, better upcast and
shift as unsigned.

Change-Id: I068013a10e18043c1534c7c61be8ff8a5556d460

filter/source/xsltfilter/OleHandler.cxx

@@ -117,8 +117,14 @@ namespace XSLT
             {
                 return "Can not read the length.";
             }
-        int oleLength = (aLength[0] << 0) + (aLength[1] << 8)
-                + (aLength[2] << 16) + (aLength[3] << 24);
+        sal_Int32 const oleLength = (static_cast<sal_uInt8>(aLength[0]) <<  0U)
+                                  | (static_cast<sal_uInt8>(aLength[1]) <<  8U)
+                                  | (static_cast<sal_uInt8>(aLength[2]) << 16U)
+                                  | (static_cast<sal_uInt8>(aLength[3]) << 24U);
+        if (oleLength < 0)
+        {
+            return "invalid oleLength";
+        }
         Sequence<sal_Int8> content(oleLength);
         //Read all bytes. The compressed length should less then the uncompressed length
         readbytes = subStream->readBytes(content, oleLength);