crypto/asn1/a_time.c: Add check for OPENSSL_malloc

crypto/asn1/a_time.c: Add check for OPENSSL_malloc

As the potential failure of the OPENSSL_malloc(),
timestamp_tm could be NULL and be used in ASN1_TIME_to_tm()
without check.
Therefore, it should be better to check the return value of
OPENSSL_malloc() and return error if fails.

Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Todd Short <todd.short@me.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18575)