BUG/MINOR: quic: SIGFPE in quic_cubic_update()

BUG/MINOR: quic: SIGFPE in quic_cubic_update()

As reported by @Tristan971 in GH #2116, the congestion control window could be zero
due to an inversion in the code about the reduction factor to be applied.
On a new loss event, it must be applied to the slow start threshold and the window
should never be below ->min_cwnd (2*max_udp_payload_sz).

Same issue in both newReno and cubic algorithm. Furthermore in newReno, only the
threshold was decremented.

Must be backported to 2.6 and 2.7.