[net] Remove assert(nMaxInbound > 0)
[net] Remove assert(nMaxInbound > 0)

nMaxInbound might very well be 0 or -1, if the user prefers to keep
a small number of maxconnections.

Note: nMaxInbound of -1 means that the user set maxconnections
to 8 or less, but we still want to keep an additional slot for
the feeler connection.

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