ipfix: Fix SIGFPE in bridge exporter sampling.

ipfix: Fix SIGFPE in bridge exporter sampling.

A divide-by-zero exception like the below could occur when IPFIX
configuration is cleared while handling sampled packets from the
datapath. While it's not valid to configure the sampling probability of
IPFIX to zero via explicitly setting it in OVSDB, it is possible to
clear the configuration, which results in a probability of zero. In this
case, there is a window during which it is possible for upcalls to find
the cleared IPFIX object and attempt to perform sampling using it. Fix
the issue by ensuring that the probability is nonzero before using it.

"Program terminated with signal SIGFPE, Arithmetic exception."

dpif_ipfix_bridge_sample (...) at ../ofproto/ofproto-dpif-ipfix.c:1701
process_upcall (...) at ../ofproto/ofproto-dpif-upcall.c:1145
recv_upcalls (...) at ../ofproto/ofproto-dpif-upcall.c:705
udpif_upcall_handler (...) at ../ofproto/ofproto-dpif-upcall.c:631
ovsthread_wrapper (...) at ../lib/ovs-thread.c:340
start_thread (...) at pthread_create.c:312
clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Romain Lenglet <romain.lenglet@oracle.com>