Fix MMD0-3 Soundtracker compatible slides

MED MMD0/1/2/3 "Soundtracker compatible slides" should activate pitch
slides in all frames, so they're not really Soundtracker compatible.
Fixes synth-a-sysmic.med, reported by Simon Spiers <srspiers@gmail.com>.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Refactor code to use clamp macro

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>