extra sanity check for habitat types

src/cats/overlays/overlay_habitat_type_cc.c

@@ -107,8 +107,9 @@ double get_habitat_cc_multiplier(const struct habitat_layer_cc_aux *aux, int32_t
                             aux->max_code);
                 exit(EXIT_FAILURE);
         }
-
-        return aux->habitat_multipliers[habitat_type];
+        double result = aux->habitat_multipliers[habitat_type];
+        assert(result >= 0.0);
+        return result;
 }