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.
>>   
>Could you propose a fix?

I fixed it in my copy by simply removing the check on windows' negative
values.  If you want, I can update the doc strings, too.  It is a very
quick fix.

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