Fix xread() return value checking

The clang compiler noticed the following problem:

  tools/test-itch41.c:71:23: error: comparison of unsigned expression < 0 is
      always false [-Werror,-Wtautological-compare]
        if (stream->avail_in < 0)
            ~~~~~~~~~~~~~~~~ ^ ~

Fix it up.

Signed-off-by: Pekka Enberg <penberg@kernel.org>