A new LJ bug has people randomly logged in to the wrong accounts
There doesn't seem to be much you can do about this and the chances of someone actually hostile or known to you getting logged in to your account must be extremely low. However if its vitally important to you that no random stranger gets to see your locked posts then you might want to consider deleting them...
It also might be a good moment to double check you have all your account recovery information set properly (if you no longer have access to the email address you used when you created the journal, I understand recovery can be tricky, so if you don't it may be worth checking out what you would need) and if you back up your journal somewhere then, again, possibly not a bad moment to make sure you have an up-to-date backup. It doesn't hurt to do that kind of thing from time to time anyway and this way, if you're extremely unlucky and someone malicious does get in, change your password and delete your entries, you will be able to get your account back and you won't lose any information.
There doesn't seem to be much you can do about this and the chances of someone actually hostile or known to you getting logged in to your account must be extremely low. However if its vitally important to you that no random stranger gets to see your locked posts then you might want to consider deleting them...
It also might be a good moment to double check you have all your account recovery information set properly (if you no longer have access to the email address you used when you created the journal, I understand recovery can be tricky, so if you don't it may be worth checking out what you would need) and if you back up your journal somewhere then, again, possibly not a bad moment to make sure you have an up-to-date backup. It doesn't hurt to do that kind of thing from time to time anyway and this way, if you're extremely unlucky and someone malicious does get in, change your password and delete your entries, you will be able to get your account back and you won't lose any information.