openssl rehash: add check for OPENSSL_strdup

openssl rehash: add check for OPENSSL_strdup

As the potential failure of the memory allocation,
it should be better to check the return value of
OPENSSL_strdup() and return error if fails.
Also, we need to restore the 'ep' to be NULL if fails.

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/17676)