ng_pipe: Replace deprecated random() with prng32_bounded()

Signed-off-by: Martin Vahlensieck <git@academicsolutions.ch>

MFC after: 2 weeks
Reviewed by: markj
Pull Request: https://github.com/freebsd/freebsd-src/pull/1181

(cherry picked from commit a3ecf8c7863683e53c77a17f96ab7012352265b9)