tests: Avoid fuzzer-specific nullptr dereference in libevent when handling PROXY requests
tests: Avoid fuzzer-specific nullptr dereference in libevent when handling PROXY requests

Summary:
```
The dereference (req->evcon->http_server) takes place in
evhttp_parse_request_line and is a consequence of our hacky but
necessary use of the internal function evhttp_parse_firstline_ in the
http_request fuzzing harness.

The suggested workaround is not aesthetically pleasing, but it
successfully avoids the troublesome code path.

" http:// HTTP/1.1\n" was a crashing input prior to this workaround.
```

Backport of core [[https://github.com/bitcoin/bitcoin/pull/19140 | PR19140]].

Depends on D9001.

Test Plan:
  ninja bitcoin-fuzzers
  echo " http:// HTTP/1.1" > input
  ./src/test/fuzz/http_request input

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9002

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