Check the return from OPENSSL_buf2hexstr()

The function OPENSSL_buf2hexstr() can return NULL if it fails to allocate
memory so the callers should check its return value.

Fixes #10525

Reported-by: Ziyang Li (@Liby99)

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/10526)
5 files changed