Fix all memory leaks reported by valgrind

src/game.c

@@ -162,6 +162,7 @@ static void shuffle_deck(struct deck *deck) {
   for (int i = 0; i < NUMBER_OF_CARDS; i++) {
     stack_push(&(deck->stock), card[i]);
   }
+  free(card);
 }
 
 static void deal_cards(struct deck *deck) {

src/stack.c

@@ -69,6 +69,7 @@ int stack_length(struct stack *stack) {
 void stack_push(struct stack **stack, struct card *card) {
   if (card) {
     if (stack_empty(*stack)) {
+      card_free((*stack)->card);
       (*stack)->card = card;
     } else {
       /* Allocating by hand because stack_malloc() would