tcp: fix sending RST after second inp lookup

When we first find an inp, we set also the tp. If then a second
lookup is necessary, the inp is recomputed. If this fails, the
tp is not cleared, which resulted in failing KASSERT.
Therefore, clear the tp when staring the inp lookup procedure.
Reported by: Jenkins
Fixes: 02d15215cef2 ("tcp: improve blackhole support")
MFC after: 1 week
Sponsored by: Netflix, Inc.