crypt: fix nil pointer panic in ObjectInfo.Hash - fixes #10173

OverrideRemote.UnWrap returns nil when the wrapped ObjectInfo is not an
fs.Object, as when serve s3 uploads through a combine remote over crypt.
Hash then called Fs() on that nil object and panicked. Return an empty
hash instead, as for other sources which cannot be unwrapped.

backend/crypt/crypt.go

@@ -1197,7 +1197,7 @@ func (o *ObjectInfo) Hash(ctx context.Context, hash hash.Type) (string, error) {
 		return "", nil
 	}
 	// if this is wrapping a local object then we work out the hash
-	if srcObj.Fs().Features().IsLocal {
+	if srcObj != nil && srcObj.Fs().Features().IsLocal {
 		// Read the data and encrypt it to calculate the hash
 		fs.Debugf(o, "Computing %v hash of encrypted source", hash)
 		return o.f.computeHashWithNonce(ctx, o.nonce, srcObj, hash)

backend/crypt/crypt_internal_test.go

@@ -93,6 +93,16 @@ func testObjectInfo(t *testing.T, f *Fs, wrap bool) {
 	assert.Equal(t, fmt.Sprintf("%x", wantHash), gotHash)
 }
 
+// Test ObjectInfo.Hash doesn't panic when the source is an
+// fs.OverrideRemote which wraps an fs.ObjectInfo that isn't an fs.Object
+func TestObjectInfoHashOverrideRemoteNotObject(t *testing.T) {
+	src := object.NewStaticObjectInfo("file", time.Now(), 3, true, nil, nil)
+	oi := (&Fs{}).newObjectInfo(fs.NewOverrideRemote(src, "new_remote"), nonce{})
+	gotHash, err := oi.Hash(context.Background(), hash.MD5)
+	require.NoError(t, err)
+	assert.Equal(t, "", gotHash)
+}
+
 func testComputeHash(t *testing.T, f *Fs) {
 	var (
 		contents = random.String(100)