TCP PRR: Patch div/0 in tcp_prr_partialack

With clearing of recover_fs in bc7ee8e5bc555, div/0
was observed while processing partial_acks.

Suspect that rewind of an erraneous RTO may be
causing this - with the above change, recover_fs
would no longer retained at the last calculated
value, and reset. But CC_RTO_ERR can reenable
IN_RECOVERY(), without setting this again.

Adding a safety net prior to the division in that
function, which I missed in D28114.