Moments of MixtureModel no longer includes components with 0 probability

If, some mixture components have 0 probability, but moments that are Inf or NaN, e.g.
```julia
Truncated(Normal(), 10, Inf)
```
then this can cause the mixture moments to be Inf or NaN, even when they finite.

Here, we fix this, by not incorporating mixture components with 0 probability in the moment computation.