Include assert.h.

Include assert.h.
(fillrand): Add a parameter, size_max.
Add an assertion.
Adjust caller.
(dopass): Break out of the `for (;;)' loop if size < offset.
That can happen now that dopass is called with SIZE == -1.
(do_wipefd): Accept a length of zero only for a regular file.
If lseek fails or returns 0 for a non-regular file, let dopass
determine the length.
Inspired by a patch from Alan Iwi.