Fix callback check

We need to check for getCrossSisgningKey but that was added
unconditionally elsewhere - only add it if we actually have a
getSecretStorageKey callback to use.

src/client.js

@@ -1506,7 +1506,7 @@ MatrixClient.prototype.createKeyBackupVersion = async function(info) {
     await this._crypto._signObject(data.auth_data);
 
     if (
-        this._cryptoCallbacks.getSecretStorageKey &&
+        this._cryptoCallbacks.getCrossSigningKey &&
         this._crypto._crossSigningInfo.getId()
     ) {
         // now also sign the auth data with the cross-signing master key

src/crypto/index.js

@@ -218,7 +218,7 @@ export default function Crypto(baseApis, sessionStore, userId, deviceId,
     );
 
     // Assuming no app-supplied callback, default to getting from SSSS.
-    if (!cryptoCallbacks.getCrossSigningKey) {
+    if (!cryptoCallbacks.getCrossSigningKey && cryptoCallbacks.getSecretStorageKey) {
         cryptoCallbacks.getCrossSigningKey = async (type) => {
             return CrossSigningInfo.getFromSecretStorage(type, this._secretStorage);
         };