buffer: use memcpy in buf_catrunc

buffer: use memcpy in buf_catrunc

Since we use strlen() to determine the length
and then check it ourselves, there is really
no point in using strncpy.

But the compiler might complain that we use
the output of strlen() for the length of
strncpy which is usually a sign for bugs:

error: ‘strncpy’ specified bound depends
 on the length of the source argument
 [-Werror=stringop-overflow=]

Warning was at least triggered for
mingw-gcc version 10-win32 20220113.

Also change the type of len to size_t
which avoids potential problems with
signed overflow.

v2:
 - make len size_t and change code to avoid any theoretical overflows
 - remove useless casts
v3:
 - fix off-by-one introduced by v2 %)
v4:
 - ignore unsigned overflow to simplify code

Change-Id: If4a67adac4d2e870fd719b58075d39efcd67c671
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20230517113619.82808-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26683.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>