Fixed potential use-after-free on detached threads

This could sometimes cause REAPER's plugin scanning to crash when the
stars aligned in the wrong way since the stop token would no longer
exist.

CHANGELOG.md

@@ -10,6 +10,9 @@ Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### Fixed
 
+- Fixed a potential crash that could happen if the host would unload a plugin
+  immediately after its initialization. This issue affected the plugin scanning
+  in _REAPER_.
 - Fixed parsing order of `yabridge.toml`. Sections were not always read from top
   to bottom like they should be, which could cause incorrect setting overrides.
 - Fixed an initialization error when using plugin groups for plugins that are

src/plugin/plugin-bridge.cpp

@@ -91,7 +91,7 @@ PluginBridge::PluginBridge(audioMasterCallback host_callback)
     // when it is not. The alternative would be to rewrite this to using
     // `async_accept`, Boost.Asio timers, and another IO context, but I feel
     // like this a much simpler solution.
-    std::jthread host_guard_handler([&](std::stop_token st) {
+    host_guard_handler = std::jthread([&](std::stop_token st) {
         using namespace std::literals::chrono_literals;
 
         while (!st.stop_requested()) {
@@ -102,7 +102,7 @@ PluginBridge::PluginBridge(audioMasterCallback host_callback)
                 std::terminate();
             }
 
-            std::this_thread::sleep_for(1s);
+            std::this_thread::sleep_for(20ms);
         }
     });
 #endif
@@ -118,7 +118,6 @@ PluginBridge::PluginBridge(audioMasterCallback host_callback)
 
 #ifndef WITH_WINEDBG
     host_guard_handler.request_stop();
-    host_guard_handler.detach();
 #endif
 
     // There's no need to keep the socket endpoint file around after accepting

src/plugin/plugin-bridge.h

@@ -208,6 +208,15 @@ class PluginBridge {
      */
     std::unique_ptr<HostProcess> vst_host;
 
+    /**
+     * A thread used during the initialisation process to terminate listening on
+     * the sockets if the Wine process cannot start for whatever reason. This
+     * has to be defined here instead of in the constructor we can't simply
+     * detach the thread as it has to check whether the VST host is still
+     * running.
+     */
+    std::jthread host_guard_handler;
+
     /**
      * Whether this process runs with realtime priority. We'll set this _after_
      * spawning the Wine process because from my testing running wineserver with