Add a second KASSERT checking for len >= 0 in the tcp output path.
Add a second KASSERT checking for len >= 0 in the tcp output path.

This is different to the first one (as len gets updated between those
two) and would have caught various edge cases (read bugs) at a well
defined place I had been debugging the last months instead of
triggering (random) panics further down the call graph.

MFC after:	2 months

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