apps/ocsp: Add check for OPENSSL_strdup

apps/ocsp: Add check for OPENSSL_strdup

Just assert 'bn' to be non-NULL is not enough.
The check for 'itmp' is still needed.
If 'bn' is 0, the 'itmp' is assigned by OPENSSL_strdup().
Since OPENSSL_strdup() may fail because of the lack of memory,
the 'itmp' will be NULL and be an valid parameter hashed in
TXT_DB_get_by_index(), returning a wrong result.

Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17677)