avoid UB where mres is optimized away

Index: mda/exmdb_local/exmdb_local.cpp
--- mda/exmdb_local/exmdb_local.cpp.orig
+++ mda/exmdb_local/exmdb_local.cpp
@@ -281,6 +281,7 @@ delivery_status exmdb_local_deliverquota(MESSAGE_CONTE
 		return delivery_status::no_user;
 	}
 	auto home_dir = mres.maildir.c_str();
+	mlog(LV_DEBUG, "<%s> mailbox is in %s", address, home_dir);
 	auto pmail = &pcontext->mail;
 	bool deliver_to_junk = should_move_to_junk(*pmail);
 	gx_strlcpy(hostname, get_host_ID(), std::size(hostname));
