In PKINIT, check for null PKCS7 enveloped fields

The PKCS7 ContentInfo content field and EncryptedContentInfo
encryptedContent field are optional.  Check for null values in
cms_envelopeddata_verify() before calling pkcs7_decrypt().  Reported
by Bahaa Naamneh.

(cherry picked from commit 48ccd81656381522d1f9ccb8705c13f0266a46ab)

https://github.com/krb5/krb5/commit/5cbfb6366f6c07903e67c87db0144d280717cf0c
Author: Greg Hudson <ghudson@mit.edu>
Commit: 5cbfb6366f6c07903e67c87db0144d280717cf0c
Branch: krb5-1.21
 src/plugins/preauth/pkinit/pkinit_crypto_openssl.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)