Fix silent failure in ASYNC_start_job when size is 0

Fix silent failure in ASYNC_start_job when size is 0

When ASYNC_start_job is called with args != NULL but size == 0,
OPENSSL_malloc(0) is called. Depending on the libc implementation,
malloc(0) may return NULL, causing a silent failure.

This patch modifies the logic to skip allocation if size is 0.

CLA: trivial

Reviewed-by: Norbert Pocs <norbertp@openssl.org>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/29377)