Fix centralF returning NaN
Fix centralF returning NaN

When x = 0 and df1 <= 2, the pdf was still returning NaN. There were a
couple of checks in the code that appeared to be intended to deal with
this situation, but they weren't conditional on x = 0, when they should,
and they were conditional on the value of df2, when they shouldn't. This
fixes those problems.

PR-URL: https://github.com/jstat/jstat/pull/153
Reviewed-by: Trevor Norris <9cb5491eb5770dd8e0bf77a8a0335abf5f0488c5@gmail.com>

akrawitz/jstat,ggonzale/jstat,NorthDecoder/jstat,jstat/jstat,akrawitz/jstat,jstat/jstat,ggonzale/jstat,NorthDecoder/jstat