Just keep in mind that gun powder is hydroscopic, that is, it attracts water. If you live in a high humidity environment then you won't want to leave it that way for very long unless you take steps to keep the water out, and several ways of doing that have been pointed out already. Leaving the powder and ball charge in overnight after an unsuccessful hunt is perfectly alright, just be sure to un-cap the nipple. The only thing you might have to worry about in case you leave your rifle that way for several weeks is the powder getting a little weak from absorbing water, but it's not going to invite rust and corrosion. Properly sealed, traditional gun powder will last for many years, however, substitute powders get weaker over time, and it wouldn't be a good idea to rely on them if stored in your rifle for many months.
Hope this helps.