Add NULL check in i2d_PrivateKey()

Originally submitted by Kurt Cancemi <kurt@x64architecture.com>

Closes RT#4533
Reviewed-by: NMatt Caswell <matt@openssl.org>

crypto/asn1/i2d_pr.c

@@ -69,10 +69,13 @@ int i2d_PrivateKey(EVP_PKEY *a, unsigned char **pp)
     }
     if (a->ameth && a->ameth->priv_encode) {
         PKCS8_PRIV_KEY_INFO *p8 = EVP_PKEY2PKCS8(a);
-        int ret = i2d_PKCS8_PRIV_KEY_INFO(p8, pp);
+        int ret = 0;
+        if (p8 != NULL) {
+            ret = i2d_PKCS8_PRIV_KEY_INFO(p8, pp);
             PKCS8_PRIV_KEY_INFO_free(p8);
+        }
         return ret;
     }
     ASN1err(ASN1_F_I2D_PRIVATEKEY, ASN1_R_UNSUPPORTED_PUBLIC_KEY_TYPE);
-    return (-1);
+    return -1;
 }