Added init function

Issue : Init/create function

stack.c

@@ -7,7 +7,6 @@
 
 void stack_init(stack *s)
 {
-    assert(s->data!=NULL && "Error : stack already in memory");
     s->top = -1;
     s->capacity = DEFAULT_CAPACITY;
     s->data = malloc(sizeof(int) * DEFAULT_CAPACITY);