CID 22554: Improper use of negative value (REVERSE_NEGATIVE)

I believe this was a false positive, but more asserts can never hurt.

src/kernel/unit.c

@@ -735,6 +735,7 @@ variant read_unit_reference(struct storage * store)
 
 int get_level(const unit * u, skill_t id)
 {
+    assert(id != NOSKILL);
     if (skill_enabled(id)) {
         skill *sv = u->skills;
         while (sv != u->skills + u->skill_size) {

src/skill.c

@@ -45,6 +45,7 @@ const char *skillnames[MAXSKILLS] = {
 bool skill_disabled[MAXSKILLS];
 
 bool skill_enabled(skill_t sk) {
+    assert(sk != NOSKILL);
     return !skill_disabled[sk];
 }