[fabifont]

hijackLocked marks the connection hijacked and clears its deadlines
before the Peek that collects the background read's byte into the
bufio.Reader. If that Peek fails, Hijack returns an error and no
connection, but the connection stays marked hijacked: conn.serve then
returns without closing it, its ConnState hook never sees
StateHijacked or StateClosed, and the net.Conn is only released when
the garbage collector finalizes its descriptor.

Mark the connection hijacked, and hand over its deadlines and
writer, only once the byte has been collected, so that a failed
hijack leaves the connection with the server as if Hijack had not
been called, and the server closes it as usual. The Peek cannot fail
with the readers the server sets up today, so this fixes the error
path rather than a case anyone has hit.

Fixes #82385

[gopherbot]

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