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.
struct jv_parser* jv_parser_new(int flags) {
  struct jv_parser* p = jv_mem_alloc(sizeof(struct jv_parser));
  parser_init(p, flags);
  p->flags = flags;
  return p;
}

struct jv_parser* jv_parser_new(int flags) {
  struct jv_parser* p = jv_mem_alloc(sizeof(struct jv_parser));
  parser_init(p, flags);
  p->flags = flags;
  return p;
}
