speed: Fix memory leak

Free the signature stack after iterating over all found signatures.
Free the kem and signature stacks at the end of speed_main() if not
NULL.

Signed-off-by: Ingo Franzki <ifranzki@linux.ibm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21491)

(cherry picked from commit cc7e2b20de02959c328f96e464e5fb8b256a00e0)