util/charset/convert: do not overflow dest len in corner case

util/charset/convert: do not overflow dest len in corner case

Now, if destlen were SIZE_MAX - 1, destlen * 2 would wrap to SIZE_MAX - 3,
which makes (destlen * 2 + 2) == SIZE_MAX - 1, the same number again.
So we need the <= comparison in this case.

As things stand, it is not actually possible for destlen to be
SIZE_MAX (because it is always an even number after the first round,
and the first round is constrained to be < SIZE_MAX / 2, but *if*
destlen was SIZE_MAX, destlen * 2 + 2 would be 0, so that case is OK.
Similarly the SIZE_MAX - 2 and smaller cases were covered by the
original formula.

We add the comment for people who are wondering WTF is going on with
all this destlen manipulation.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>