Segfault during `cleanup_server` for bidirectional or with parallel stream tests ended early

# Context

* Version of iperf3: 3.16

* Hardware: N/A

* Operating system (and distribution, if any): 6.5.0-26-generic # 26~22.04.1-Ubuntu

* Other relevant information (for example, non-default compilers,
libraries, cross-compiling, etc.): N/A

# Bug Report

While doing some testing I would occasionally use the wrong iperf flags/parameters and would terminate the test early rather than waiting for it to run completely.

* Expected Behavior: Terminating a test early causes the client and server to stop testing. The client cleans up and terminates. The server cleans up and prepares for next test.

* Actual Behavior: Server segfaults during cleanup

* Steps to Reproduce
1. Simulate a high-ish latency link on the loopback interface: `tc qdisc add dev lo root netem delay 50ms`
2. Start server: `iperf3 -s`
3. Start client and terminate test early: `iperf3 -c 127.0.0.1 -t 10 -P 10` or `iperf3 -c 127.0.0.1 -t 10 --bidir`
* It seems to be a race condition so to have better chances at finding it I'll typically run something like `for i in $(seq 100); do iperf3 -c 127.0.0.1 -t 10 -P 10; done` then repeatedly use ctrl-c to kill tests.
4. Check server crashed

* Possible Solution
Adding an assert into the code [here](https://github.com/esnet/iperf/blob/master/src/iperf_server_api.c#L421) shows the root cause. Something like `assert(sp->thr != 0);`. This would indicate that a NULL values is being passed into `pthread_cancel`. A possible solution would be a NULL check before attempting to cancel the thread.

* Other observations
I was not able to reproduce the issue using 3.15 as the server.