blackman and flattopwin cannot be used with pwelch

[Francesco Potorti`]

pwelch.m as distributed in the signal package of octave-forge does not
accept a window with negative values.  However, blackman(4096) and
flattopwin(4096) generate negative values.

I suspect that as far as blackman is concerned, it is just a rounding
error, but flattopwin definitely generates negative values.

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