iservice: fix calculation of converted domains size

iservice: fix calculation of converted domains size

To keep track of how much of the buffer is already used, the difference
of the current position and the start of the buffer needs to be
multiplied with the size of a character / glyph to bet the byte count,
with which calculations are done further down below.

Reported-by: Marc Heuse <marc@srlabs.de>
Reported-by: stephan@srlabs.de
Change-Id: I16f9426e57f4802ba038ab51f5b70161464b9428
Signed-off-by: Heiko Hund <heiko@ist.eigentlich.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1390
Message-Id: <20251126103427.4085-1-gert@greenie.muc.de>
Signed-off-by: Gert Doering <gert@greenie.muc.de>