Skill levels display -1 rather than next level when level is disabled
When using custom skill levels with some level disabled (f.e skill_medic "20 -1 90 -1" ), a few minor issues appear:
- In the stats window, "Next level" is displayed as -1 (ie, 23/-1). We should rather display the next available level (ie, 90), or nothing at all if the last level isn’t available
- Skill levels stars are still displayed (in-game and debriefing screen), even if the intermediary level was not available. We should grey it out instead.