utils/fifo8: change fatal errors from abort() to assert()

Developer errors are better represented with assert() rather than abort(). Also
improve the strictness of the checks by using range checks within the assert()
rather than converting the existing equality checks to inequality checks.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Claudio Fontana <cfontana@suse.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210121102518.20112-1-mark.cave-ayland@ilande.co.uk>