tests: Avoid fuzzer-specific nullptr dereference in libevent when handling PROXY requests
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 PR19140.
Depends on D9001.
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