Assuming there are no problems with the iPhone at the moment you should be able to use Edit > Preferences > Devices to delete the corrupt backup set, then backup your iPhone. No data should be lost. I'd recommend you restart your iPhone first to reduce the possiblity that something is going wrong at that end during the backup.
Note you should probably aim to backup your iPhone every week, or even more frequently. A year's worth of data is a lot to lose. It is also worth understanding that an iOS backup is a "rolling backup". Each time you update the backup it becomes a snapshot of the current state of the device (not including media which should be in your library). If you realise too late that you've lost some content from it you can't step back through earlier versions of the backup data. The way to create a permanent backup point that could be used again and again is to restore the device immediately after backing up.