--get-server-output option does not work when using option --udp-counters-64bit

* Version of iperf3: 3.1.3

* Hardware: Ubuntu 16.04

# Bug Report
When the client is run with '--get-server-output', '--udp-counters-64bit', '--json' option and '--json' option on server, the client gets **always** runs into an error "Error - unable to receive results" for server output.

* Expected Behavior
The client should have received the server output json after the completion of the test.

* Actual Behavior
The client gets an error "Error - unable to receive results" for server output.

* Steps to Reproduce
On the server, run "iperf3 -s --json"
On the client, run "iperf3 -c server_ip --json -u -w 4m -i 1 --bandwidth 1000000000 -t 60 --length 30000 --get-server-output --udp-counters-64bit"