trivial: initialize the dirty buffer with a random-ish byte.
Stops valgrind from whining about uninitialized buffers.
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>