bio_enc.c: add check for BIO_new_mem_buf

bio_enc.c: add check for BIO_new_mem_buf

Since the memory allocation may fail, the BIO_new_mem_buf() may
return NULL pointer.
Therefore, it should be better to check it and return error if fails.

Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17730)