Heap buffer overflow in tokenadd() (fix #105)

This was an off-by one: the NUL terminator byte was not allocated on
resize.  This was triggered by JSON-encoded numbers longer than 256
bytes.
static void push(struct jv_parser* p, jv v) {
  assert(p->stackpos <= p->stacklen);
  if (p->stackpos == p->stacklen) {
    p->stacklen = p->stacklen * 2 + 10;
    p->stack = jv_mem_realloc(p->stack, p->stacklen * sizeof(jv));
  }
  assert(p->stackpos < p->stacklen);
  p->stack[p->stackpos++] = v;
}

static void push(struct jv_parser* p, jv v) {
  assert(p->stackpos <= p->stacklen);
  if (p->stackpos == p->stacklen) {
    p->stacklen = p->stacklen * 2 + 10;
    p->stack = jv_mem_realloc(p->stack, p->stacklen * sizeof(jv));
  }
  assert(p->stackpos < p->stacklen);
  p->stack[p->stackpos++] = v;
}
