… in libevent when handling PROXY requests

20d31bd tests: Avoid fuzzer-specific nullptr dereference in libevent when handling PROXY requests (practicalswift)

Pull request description:

  Avoid constructing requests that will be interpreted by libevent as PROXY requests to avoid triggering a `nullptr` dereference. Split out from bitcoin#19074 as suggested by MarcoFalke.

  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.

  Before this PR:

  ```
  $ echo " http:// HTTP/1.1" > input
  $ src/test/fuzz/http_request input
  src/test/fuzz/http_request: Running 1 inputs 1 time(s) each.
  Running: input
  AddressSanitizer:DEADLYSIGNAL
  =================================================================
  ==27905==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000108 (pc 0x55a169b7e053 bp 0x7ffd452f1160 sp 0x7ffd452f10e0 T0)
  ==27905==The signal is caused by a READ memory access.
  ==27905==Hint: address points to the zero page.
      #0 0x55a169b7e053 in evhttp_parse_request_line depends/work/build/x86_64-pc-linux-gnu/libevent/2.1.11-stable-36daee64dc1/http.c:1883:37
      #1 0x55a169b7d9ae in evhttp_parse_firstline_ depends/work/build/x86_64-pc-linux-gnu/libevent/2.1.11-stable-36daee64dc1/http.c:2041:7
      #2 0x55a1687f624e in test_one_input(std::vector<unsigned char, std::allocator<unsigned char> > const&) src/test/fuzz/http_request.cpp:51:9
  …
  $ echo $?
  1
  ```

  After this PR:

  ```
  $ echo " http:// HTTP/1.1" > input
  $ src/test/fuzz/http_request input
  src/test/fuzz/http_request: Running 1 inputs 1 time(s) each.
  Running: input
  Executed input in 0 ms
  ***
  *** NOTE: fuzzing was not performed, you have only
  ***       executed the target code on a fixed set of inputs.
  ***
  $ echo $?
  0
  ```

  See [`doc/fuzzing.md`](https://github.com/bitcoin/bitcoin/blob/master/doc/fuzzing.md) for information on how to fuzz Bitcoin Core. Don't forget to contribute any coverage increasing inputs you find to the [Bitcoin Core fuzzing corpus repo](https://github.com/bitcoin-core/qa-assets).

  Happy fuzzing :)

Top commit has no ACKs.

Tree-SHA512: 7a6b68e52cbcd6c117487e74e47760fe03566bec09b0bb606afb3b652edfd22186ab8244e8e27c38cef3fd0d4a6c237fe68b2fd22e0970c349e4ab370cf3e304

…dling 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 [[bitcoin/bitcoin#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