tests: Avoid fuzzer-specific nullptr dereference in libevent when handling PROXY requests #19140

Avoid constructing requests that will be interpreted by libevent as PROXY requests to avoid triggering a nullptr dereference. Split out from #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](/bitcoin-bitcoin/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](/bitcoin-bitcoin/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](/bitcoin-bitcoin/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 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.

Happy fuzzing :)

@theStack you might be interested in this?

Any chance of getting this merged? It is a bit cumbersome to wade through tons of spurious http_request crashes when fuzzing Bitcoin Core master :)

Note that this is a workaround for a bug (of mine) in the fuzzing harness -- not in libevent -- so should be entirely safe to merge. Touches only src/test/fuzz/http_request.cpp.