Unbreak usr.bin/dc with OpenSSL 3

OpenSSL's BN_zero API has had a void return value since 1.1.x: chase the
change to unbreak the build on OpenSSL 3.x with MK_GH_BC == no.

MFC after: 2 weeks
Reviewed by: emaste, kevans
Differential Revision: https://reviews.freebsd.org/D41410