Fix NULL deref in apps/pkcs7

Thanks to Brian Carpenter for finding and reporting this.

Reviewed-by: Emilia Käsper <emilia@openssl.org>

apps/pkcs7.c

diff --git a/apps/pkcs7.c b/apps/pkcs7.c
index ad8330d..a2c1c6d 100644
--- a/apps/pkcs7.c
+++ b/apps/pkcs7.c
@@ -222,12 +222,16 @@
         i = OBJ_obj2nid(p7->type);
         switch (i) {
         case NID_pkcs7_signed:
-            certs = p7->d.sign->cert;
-            crls = p7->d.sign->crl;
+            if (p7->d.sign != NULL) {
+                certs = p7->d.sign->cert;
+                crls = p7->d.sign->crl;
+            }
             break;
         case NID_pkcs7_signedAndEnveloped:
-            certs = p7->d.signed_and_enveloped->cert;
-            crls = p7->d.signed_and_enveloped->crl;
+            if (p7->d.signed_and_enveloped != NULL) {
+                certs = p7->d.signed_and_enveloped->cert;
+                crls = p7->d.signed_and_enveloped->crl;
+            }
             break;
         default:
             break;