randomR could produce NaNs when the upper bound is infinity

When the upper bound is infinity, NaNs could be produced. For example,

```haskell
filter (\x-> x/= (1/0)) $ randomRs ((0::Float), 1/0) $ mkStdGen 0
[NaN,NaN,NaN
```

Although it's unclear what should be produced when the upper bound is infinity, I don't think NaNs should be there anyways.