Use a fixed lenght buffer in writePseudoRandomBytes()

This will reduce memory usage.

License: MIT
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
```Go
bufsize := int64(4096)
b := make([]byte, bufsize)

for count > 0 {
  if bufsize > count {
    bufsize = count
    b = b[:bufsize] // re-slice. no need to create a new buffer
  }

  for i := 0; i < bufsize; i++ {
    b[i] = byte(randmath.Intn(256))
  }
  count = count - bufsize
```