Resolve "Add pop function"

Closes #6 (closed)

stack.c

 // depile
 void stack_pop(stack *s, int *value){
-	assert(s->top>=0 && "Stcak is empty\n");	
-
+	if (stack_is_empty(*s)){
+		printf("stack is empty\n");
+		return;
+	}	
 	if (s->top == s->capacity/4){
 		s->capacity /= 2;
+		s->data = realloc(s->data, sizeof(int)*s->capacity);
 	}
-	
+
 	*value = s->data[s->top];
 	s->top -= 1;
 }
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