Resolve "Add peek function"

Closes #7 (closed)

stack.c

 #include <stdio.h>
+#include <stdlib.h>
 #include "stack.h"
 
+#define DEFAULT_CAPACITY 4
+
+void stack_init(stack *s)
+{
+    s->top = -1;
+    s->capacity = DEFAULT_CAPACITY;
+    s->data = malloc(sizeof(int) * DEFAULT_CAPACITY);
+}
+
 void stack_peek(stack s, int *value){
-    if(stack_is_empty(s)){
-        *value=s.data[s.top];
+    if (!stack_is_empty(s)) {
+        *value = s.data[s.top];
     }
 }