Return NA if either sd in the denominator is 0.

[chinandrew]

Fixes #1.

Checks if the standard deviations are close to 0 and returns NA if so, since otherwise would be dividing by 0.
Threshold I'm using is about 1.49e-8, which is based on the same threshold used by dplyr's near()

> devtools::load_all()
ℹ Loading dvmisc
> set.seed(87)
> dvmisc::sliding_cor(runif(25), rep(1.014103, 40))
 [1] NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA
> set.seed(100)
> dvmisc::sliding_cor(runif(25), rep(1.014103, 40)) 
 [1] NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA
> set.seed(100)
> dvmisc::sliding_cor(runif(30), rep(1.014103, 40)) 
 [1] NA NA NA NA NA NA NA NA NA NA NA
> set.seed(100)
> dvmisc::sliding_cor(runif(20), rep(1.014103, 40))
 [1] NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA
> set.seed(100)
> dvmisc::sliding_cor(rep(1.014103, 40), runif(50))
 [1] NA NA NA NA NA NA NA NA NA NA NA

(Included one additional example at the end if sd_shortvec is ~0, not just longvec).