app/testpmd: fix packet burst spreading stats

app/testpmd: fix packet burst spreading stats

RX/TX functions (rte_eth_rx_burst/rte_eth_tx_burst) get 'nb_pkts'
argument, which specifies the maximum number to receive/transmit.
It can be 0..nb_pkts, meaning nb_pkts+1 options.
Testpmd can provide statistics of the burst sizes ('set
record-burst-stats on') by incrementing an array cell of index
<burst-size>. This array is mistakenly [MAX_PKT_BURST] size. Receiving
the maximum burst will cause out of bound write.
Enlarge the spread stats array by one cell to fix it.

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Eli Britstein <elibr@nvidia.com>
Reviewed-by: Matan Azrad <matan@nvidia.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>