static-build: fix potential NULL dereference in openssl

`set_client_ciphersuite` can potentially dereference NULL if the session's
cipher is not set — add a check for this condition.

Closes tarantool/security#27

NO_CHANGELOG=<security fix>
NO_DOC=<security fix>
NO_TEST=<third-party security fix>

(cherry picked from commit a8c6c27c)