Compression buffer fill fix

We need to advance after the zero fill as well.

Reported-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/lib/rand.c b/lib/rand.c
index 995035f..a79fb9c 100644
--- a/lib/rand.c
+++ b/lib/rand.c
@@ -125,6 +125,8 @@
 			this_len = len;
 
 		memset(buf, 0, this_len);
+		len -= this_len;
+		buf += this_len;
 	}
 
 	return r;