In case when filesort does not use addon field packing (because of
too small potential savings) and uses fixed width addon fields instead,
the field->pack() call can store less bytes when the field maximum
possible field length, e.g. in case of VARCHAR().
The memory between the packed length and addonf->length (the maximum length)
stayed uninitialized, which was reported by Valgrind/MSAN.

The problem was introduced by f52bf920 in 10.5,
which removed the tail initialization (probably unintentionally).

Restoring the bzero() in the fixed length branch,
so in case when pack() stores less bytes than addonf->length says,
the trailing bytes gets initialized.

Note, before f52bf920, the bzero()
was under HAVE_valgrind conditional compilation. Now it's being added
unconditionally:
- MSAN also reported the problem, so it's not only Valgrind specific.
- As Serg proposed, conditional initialization is bad - it can have
  potentional security problems as the non-initialized memory fragments
  can store various pieces of essential information, e.g. passwords.