http: update libevent workaround to correct version

The libevent bug described in libevent/libevent@5ff8eb2
was already patched in release-2.1.9-beta, with cherry-picked
commits 5b40744d1581447f5b4496ee8d4807383e468e7a and
b25813800f97179b2355a7b4b3557e6a7f568df2.

There should be no side-effects by re-applying the workaround on
an already patched version of libevent, but it is best to set the
correct version number to avoid confusion.

Author: stickies-v

src/httpserver.cpp

@@ -222,8 +222,10 @@ static void http_request_cb(struct evhttp_request* req, void* arg)
         }, nullptr);
     }
 
-    // Disable reading to work around a libevent bug, fixed in 2.2.0.
-    if (event_get_version_number() >= 0x02010600 && event_get_version_number() < 0x02020001) {
+    // Disable reading to work around a libevent bug, fixed in 2.1.9
+    // See https://github.com/libevent/libevent/commit/5ff8eb26371c4dc56f384b2de35bea2d87814779
+    // and https://github.com/bitcoin/bitcoin/pull/11593.
+    if (event_get_version_number() >= 0x02010600 && event_get_version_number() < 0x02010900) {
         evhttp_connection* conn = evhttp_request_get_connection(req);
         if (conn) {
             bufferevent* bev = evhttp_connection_get_bufferevent(conn);
@@ -604,7 +606,7 @@ void HTTPRequest::WriteReply(int nStatus, const std::string& strReply)
         evhttp_send_reply(req_copy, nStatus, nullptr, nullptr);
         // Re-enable reading from the socket. This is the second part of the libevent
         // workaround above.
-        if (event_get_version_number() >= 0x02010600 && event_get_version_number() < 0x02020001) {
+        if (event_get_version_number() >= 0x02010600 && event_get_version_number() < 0x02010900) {
             evhttp_connection* conn = evhttp_request_get_connection(req_copy);
             if (conn) {
                 bufferevent* bev = evhttp_connection_get_bufferevent(conn);