The source of the problem was in my_vsnprintf() implementation.


strings/my_vsnprintf.c:
  Fixing a problem in vsnprintf('%.*s', len, ptr)
  When processing the above format, it's incorrect
  to use strlen() because the string is not necessarily
  a null terminated string.
  Changing strlen() followed by set_if_smaller()
  to strnlen() - which covers both cases - limiting
  by '\0' and by "len".