app/testpmd: fix invalid Tx queue number setting

app/testpmd: fix invalid Tx queue number setting

If an invalid number of TX queues is configured from testpmd run-time
command like "port config all txq number" or from --txq in the command
to start testpmd, the global variable nb_txq is updated by this invalid
value without this patch. It may cause testpmd crash. This patch refuses
invalid txq 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>