storage/State: fix memory leak after database mount failure

Caused by commit 64d141f71e

This wasn't a serious memory leak, because after a mount failure, MPD
would abort anyway, which is subject to the next commit.

src/storage/StorageState.cxx

@@ -104,6 +104,7 @@ storage_state_restore(const char *line, TextFile &file, Instance &instance)
 		try {
 			((SimpleDatabase *)db)->Mount(uri.c_str(), url.c_str());
 		} catch (...) {
+			delete storage;
 			throw;
 		}
 	}