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including the TCP header in the first IP packet. Enforce this in tcp_output(). In addition make sure that at least one byte payload fits in the TCP segement to allow making progress. Without this check, a kernel with INVARIANTS will panic. This issue was found by running an instance of syzkaller. Reviewed by: jtl@ MFC after: 3 days Sponsored by: Netflix, Inc. Differential Revision: https://reviews.freebsd.org/D21665