Fix an uninitialised read on an error path

Found by Coverity.
Reviewed-by: NRichard Levitte <levitte@openssl.org>

crypto/ocsp/ocsp_ext.c

@@ -257,7 +257,7 @@ static int ocsp_add1_nonce(STACK_OF(X509_EXTENSION) **exts,
      */
     os.length = ASN1_object_size(0, len, V_ASN1_OCTET_STRING);
     if (os.length < 0)
-        goto err;
+        return 0;
 
     os.data = OPENSSL_malloc(os.length);
     if (os.data == NULL)