Gix-date: gix-date: undefined behavior due to invalid string generation

A flaw was found in gix-date. The `gix_date::parse::TimeBuf::as_str` function can generate strings containing invalid non-UTF8 characters. This issue violates the internal safety invariants of the `TimeBuf` component, leading to undefined behavior when these malformed strings are subsequently processed. This could potentially result in application instability or other unforeseen consequences.