ssl: Fix -Wconversion warnings in pem_password_callback

ssl: Fix -Wconversion warnings in pem_password_callback

The OpenSSL API is how it is, so adapt with casts.

Change-Id: I053ddbb71cc5b9ae16c5a49be833035d943d7eba
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1137
Message-Id: <20250922204158.23319-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg33154.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>