pkcs11_openssl: Silence a conversion warning

pkcs11_openssl: Silence a conversion warning

The only caller of this function uses a constant
for this parameter, so this is all quite safe. Add
an ASSERT for good measure anyway to make the assumption
explicit.

Change-Id: I6079bf9e7f6b37cb2e2d7f28851a77d0b08be995
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1352
Message-Id: <20251106133936.30264-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg34209.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>