Starters usually taste pretty bad, and many strains will leave them a little tart, so unless it is really rancid, i wouldn't take a slight tartness to necessarily be a sign of infection.
Five days for it to sit is not ideal but shouldn't ruin it. There are a couple things you could do, though.
First, you could simply refrigerate. This will slow down the yeast and buy you several more days. It also has the handy benefit of causing the yeast to settle out, so you don't have to pitch that nasty starter beer into your batch. Simply take it out several hours before pitching to let it slowly warm to room temp. When your wort is cooled, decant the starter beer off and replace that with an equal amount of fresh wort. You can give it about a half hour on the stir plate to get acclimated the new environment before pitching.
Alternatively, you could just step up the starter by either going to a larger flask with more wort, or dumping about half of the starter out and adding fresh starter wort to what is left to let it repropagate to the previous cell count. This will keep the yeast active and happy until you are ready to brew.
As I said, five days just sitting there won't ruin it but might cause a slower start to your fermentation. The yeast have to have adequate glycogen reserves within their cells in order to have energy to get through the lag and growth phases smoothly prior to the uptake of wort sugars during true fermentation, and sitting around without anything to eat will deplete this over time.