db/simple: fix crash when mounting twice

The `db->close()` call was a `nullptr` dereference because the `db`
variable had already been moved.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/839

NEWS

@@ -1,6 +1,8 @@
 ver 0.21.24 (not yet released)
 * protocol
   - "tagtypes" requires no permissions
+* database
+  - simple: fix crash when mounting twice
 * fix unit test failure
 
 ver 0.21.23 (2020/04/23)

src/db/plugins/simple/SimpleDatabasePlugin.cxx

@@ -449,12 +449,7 @@ SimpleDatabase::Mount(const char *local_uri, const char *storage_uri)
 
 	// TODO: update the new database instance?
 
-	try {
 	Mount(local_uri, std::move(db));
-	} catch (...) {
-		db->Close();
-		throw;
-	}
 }
 
 inline DatabasePtr