Fix carnivores being able to eat low-cal veggies

Foods under ~17 calories, including pickles, were skipping the carnivore
check against prohibited foods.  This fix safely closes that edge case.
A comment is also included warning developers of danger from rounding.

src/consumption.cpp

@@ -465,6 +465,7 @@ std::pair<nutrients, nutrients> Character::compute_nutrient_range(
 
 int Character::nutrition_for( const item &comest ) const
 {
+    // Note: This will round down to zero for super-low-calorie foods
     return compute_effective_nutrients( comest ).kcal() / islot_comestible::kcal_per_nutr;
 }
 
@@ -901,7 +902,7 @@ ret_val<edible_rating> Character::can_eat( const item &food ) const
         return ret_val<edible_rating>::make_failure( INEDIBLE_MUTATION, _( "Ugh, you can't drink that!" ) );
     }
 
-    if( has_trait( trait_CARNIVORE ) && nutrition_for( food ) > 0 &&
+    if( has_trait( trait_CARNIVORE ) && compute_effective_nutrients( food ).kcal() > 0 &&
         food.has_any_vitamin( carnivore_blacklist ) && !food.has_flag( flag_CARNIVORE_OK ) ) {
         return ret_val<edible_rating>::make_failure( INEDIBLE_MUTATION,
                 _( "Eww.  Inedible plant stuff!" ) );