channels/cliprdr: avoid possible integer overflow

channels/cliprdr: avoid possible integer overflow

If the server sends us garbage (or the client provides it) then it is
possible for the multiplication to overflow (as it is performed on
unsigned 32-bit values) which will result in a false positive failure of
the sanity check. Avoid it by rearranging arithmetics a little.

Keep the multiplication in the error message because we are interested
in the number of bytes in the stream and how it compares to the number
we have expected based on the presumed file count.