ATS crashes when reloading parent.config

# Happened
A crash occurred while using ATS9.2.11
```
Thread 1562, [ET_NET 57]:
0 0x0000557bbf45a498 crash_logger_invoke(int, siginfo_t*, void*) + 0x88
1 0x00007fb9728d1520 __sigaction + 0x50
2 0x0000557bbf53abd2 HttpTransact::handle_request_keep_alive_headers(HttpTransact::State*, HTTPVersion, HTTPHdr*) + 0x32
3 0x0000557bbf53f039 HttpTransact::build_request(HttpTransact::State*, HTTPHdr*, HTTPHdr*, HTTPVersion) + 0xb9
4 0x0000557bbf51e584 HttpTransact::HandleCacheOpenReadHit(HttpTransact::State*) + 0x834
5 0x0000557bbf4f9941 HttpSM::call_transact_and_set_next_state(void (*)(HttpTransact::State*)) + 0x201
6 0x0000557bbf4d7cf2 HttpSM::state_api_callout(int, void*) + 0x592
7 0x0000557bbf4d3eec HttpSM::state_api_callback(int, void*) + 0xec
8 0x0000557bbf47a2fb TSHttpTxnReenable + 0x18b
9 0x00007fb96f625226 0x0 + 0x18b
```
It occurred at the moment parent.config was reloaded.

# My investigation
Based on the code and this stack trace, I suspect the issue is occurring at position [HttpTransact.cc#L6889](https://github.com/apache/trafficserver/blob/9.2.11/proxy/http/HttpTransact.cc#L6889).
`build_request` is called after the null check for [HttpTransact.cc#L2931](https://github.com/apache/trafficserver/blob/fdf5fb223c5eef92991ceb28a53cb8add956914b/proxy/http/HttpTransact.cc#L2931), but it appears there is no subsequent null check.

I would like to know the following
- Is it possible for execution to reach point [HttpTransact.cc#L6889](https://github.com/apache/trafficserver/blob/9.2.11/proxy/http/HttpTransact.cc#L6889) with `s->current.server` being null?
- Are there any other possible causes?

Any help would be appreciated