mem: check upload length before storing upload

internal/backend/mem/mem_backend.go

@@ -81,14 +81,14 @@ func (be *MemoryBackend) Save(ctx context.Context, h restic.Handle, rd restic.Re
 		return err
 	}
 
-	be.data[h] = buf
-	debug.Log("saved %v bytes at %v", len(buf), h)
-
 	// sanity check
 	if int64(len(buf)) != rd.Length() {
 		return errors.Errorf("wrote %d bytes instead of the expected %d bytes", len(buf), rd.Length())
 	}
 
+	be.data[h] = buf
+	debug.Log("saved %v bytes at %v", len(buf), h)
+
 	return ctx.Err()
 }