I agree with
@mainshipfred -- top the container up with
something to avoid oxidation and losing the entire thing. I've topped up a red wine with a bottle of white when I didn't have a better choice. I'd even use water if I had no other option (although so far, that has not happened).
One caveat -- when topping up, I use a wine that is at least as good a quality as what is in the container.
Food for thought:
At first blush, adding 1 bottle to a 5 gallon carboy, which is 4% of the volume, shouldn't make much difference -- if any. But when I read the label on red blends that list the blend ratios, I sometimes see amounts as little as 2% of a single varietal. The winemaker believes that addition makes a better wine. Given that the vast majority of these winemakers has training, experience, and/or resources far beyond mine, I'm willing to accept that the small amount in the blend makes a perceptible difference
in their wine.
These folks have better quality grapes than I can buy, so it may be that I won't see such differences in my wine, and it may also be that my palate is not good enough to really determine the difference. I also don't have hundreds of barrels of wine so that I can take 15 or 20 gallons for experimentation.