Use malloc to avoid alignment problems.

Thanks to Norm Green for reporting this issue.

Signed-off-by: Rich Salz <rsalz@openssl.org>

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5423)