Streams: Change XADD MAXLEN handling of values <= 0.
Streams: Change XADD MAXLEN handling of values <= 0.

Now a MAXLEN of 0 really does what it means: it will create a zero
entries stream. This is useful in order to make sure that the behavior
is identical to XTRIM, that must be able to reduce the stream to zero
elements when MAXLEN is given.

Also now MAXLEN with a count < 0 will return an error.

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