pkcs7: Prevent NULL pointer dereference, since sinfo is not always set.

BugLink: http://bugs.launchpad.net/bugs/1729107

commit 68a1fdbb upstream.

The ASN.1 parser does not necessarily set the sinfo field,
this patch prevents a NULL pointer dereference on broken
input.

Fixes: 99db4435 ("PKCS#7: Appropriately restrict authenticated attributes and content type")
Signed-off-by: Eric Sesterhenn <eric.sesterhenn@x41-dsec.de>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

crypto/asymmetric_keys/pkcs7_parser.c

@@ -90,6 +90,9 @@ static int pkcs7_check_authattrs(struct pkcs7_message *msg)
 	bool want;
 
 	sinfo = msg->signed_infos;
+	if (!sinfo)
+		goto inconsistent;
+
 	if (sinfo->authattrs) {
 		want = true;
 		msg->have_authattrs = true;