BUG: bug in nanops.var with ddof=1 and 1 elements would sometimes return inf rather than nan (GH6136)
closes #6136

@y-p I could not reproduce, but this seems likely the problem
 Thanks. Could this be a numpy issue? should we ping someone there?
 It comes down to precision I think;

The following 2 cases end up slightly different if the numerator is exactly 0 or not (which is why it was 'randomly' failing)

I changed it so it doesn't do the computation if the denomincator is 0, problem solved.

```
In [9]: Series(0.,index=np.arange(5)).values/0.
Out[9]: array([ nan,  nan,  nan,  nan,  nan])

In [10]: (Series(0.,index=np.arange(5)).values+0.1)/0.
Out[10]: array([ inf,  inf,  inf,  inf,  inf])
```
 That makes sense. Still, the same data behaves differently on the _same_ platform
and numpy. You have to cut fp some slack, but is this really an instance of that?

I'll open a numpy issue just in case. My numerical-fu isn't good enough to call it.
 not sure I never could reproduce
even with the same exact specs
 are you on conda? they use MKL for numerics.
 this was a grohkle 1.8 MKL build for windows
 Same here (didn't realize those were compiled against MKL). strange.
