welch_pgram index out of bounds

[goldfita]

The welch estimator isn't quite right when the segment length is greater than the input length. I'm not sure what the correct behavior is, but periodogram specifies zero padding.

> Pkg.installed("DSP")  
v"0.1.1"
> s = randn(10000)
> welch_pgram(s[1:3687],4096,3686)
ERROR: BoundsError

Anything less than or equal to noverlap results in all zeros for power.