Oh man. I am torn at the options. Half of me wants to say, just use it. The other half says its not worth it. Did you sanitize the jar before the yeast went in? If so, I wouldn't be as concerned. How hard is it to pick up another pack? Its pretty cheap stuff. I think this is a decision YOU need to make. Let us know how it goes. I know safale has a recommended shelf life after opening, and its not very long. Its on their website if you want to refer to it (I just read today, but can't remember the timeframe).
Is whats in the baby jar equivalent to a full pack? If not how much beer are you brewing and how much yeast do you have left?