tcp: prevent div by zero in cc_htcp

Make sure the divident is at least one. While cwnd should
never be smaller than t_maxseg, this can happen during
Path MTU Discovery, or when TCP options are considered
in other parts of the stack.

PR: 276674
MFC after: 3 days
Reviewed By: tuexen, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D43797