Fix building with OPENSSL_NO_TLSEXT.

Builds using no-tlsext in 1.0.0 and 0.9.8 are broken. This commit fixes the
issue. The same commit is applied to 1.0.1 and 1.0.2 branches for code
consistency. However this commit will not fix no-tlsext in those branches
which have always been broken for other reasons. The commit is not applied
to master at all, because no-tlsext has been completely removed from that
branch.

Based on a patch by Marc Branchaud <marcnarc@xiplink.com>

Reviewed-by: Emilia Käsper <emilia@openssl.org>
(cherry picked from commit 9a931208)