Fix conditional distribution plot for Negative Binomial (X) DGP

[dskinnion]

With some parameter choices, the conditional distribution plot for Negative Binomial (X) is completely gone. Other times, the graph is partially missing. (E.g. when $\beta_1 = 1.17$ and $\sigma = 1.1$, if $\beta_0 > 0.55$, there is no graph and if $0.55 > \beta > 0.16$, there is part of the graph missing.)

[dskinnion]

This also depends on n chosen, as well.