examples/bond: fix crash when there is no active slave

examples/bond: fix crash when there is no active slave

If bond_ethdev_rx_burst() called more times with no active slaves
the active slave index will point out of bounds, resulting in a
segfault.
The configured slaves needs to be checked, and if none became active
there is no point going further.

Do not start the packet processing threads until all configured
slaves become active.

Fixes: cc7e8ae84faa ("examples/bond: add example application for link bonding mode 6")
Cc: stable@dpdk.org
Signed-off-by: Radu Nicolau <radu.nicolau@intel.com>
Acked-by: Chas Williams <chas3@att.com>