app/testpmd: fix invalid Rx queue number setting

app/testpmd: fix invalid Rx queue number setting

If an invalid number of RX queues is configured from testpmd run-time
command like "port config all rxq number" or from --rxq in the command
to start testpmd, the global variable nb_rxq is updated by this invalid
value without this patch. It may cause testpmd crash. This patch refuses
invalid rxq setting and keeps its last correct value.

Fixes: ce8d561418d4 ("app/testpmd: add port configuration settings")
Cc: stable@dpdk.org
Signed-off-by: Wei Dai <wei.dai@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Tested-by: Yuan Peng <yuan.peng@intel.com>