CC Cubic: fix underflow for cubic_cwnd()

Singed calculations in cubic_cwnd() can result in negative cwnd
value which is then cast to an unsigned value. Values less than
1 mss are generally bad for other parts of the code, also fixed.

Submitted by: Jason Eggleston <jason@eggnet.com>
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D14141