utvideodec: Prevent possible signed overflow

Doing slice_end - slice_start is unsafe and can lead to undefined behavior
until slice_end has been properly sanitized.

Reviewed-by: Ronald S. Bultje <rsbultje@gmail.com>
Signed-off-by: Ganesh Ajjanagadde <gajjanag@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
