demos/cms/cms_ddec.c: Replace "in" with "dcont" to correctly check the success of...

demos/cms/cms_ddec.c: Replace "in" with "dcont" to correctly check the success of BIO_new_file()

Replace "in" with "dcont" to properly check the return value of BIO_new_file().

Fixes: 1728756 ("Detached encrypt/decrypt example, fix decrypt sample.")
Signed-off-by: Jiasheng Jiang <jiashengjiangcool@gmail.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27896)

(cherry picked from commit 8a7545607e872ccaff3018e2cd201cce65e615ec)