Fix LoadedPlugins::remove crash during static destruction (#12854)

When a PluginFactory is destroyed during static destruction (after main
exits), the EThreads are already gone. LoadedPlugins::remove() was
crashing because it tried to acquire a mutex lock using this_ethread(),
which returns nullptr during static destruction.

Now check if this_ethread() returns nullptr and handle cleanup directly
in that case, since we're exiting anyway and don't need the mutex
protection or scheduled deletion.

This issue was discovered while testing the header_rewrite run-plugin
feature, which creates a PluginFactory that persists as a static object
and is destroyed during static destruction.

Author: bneradt

src/proxy/http/remap/PluginDso.cc

@@ -329,10 +329,18 @@ PluginDso::LoadedPlugins::add(PluginDso *plugin)
 void
 PluginDso::LoadedPlugins::remove(PluginDso *plugin)
 {
-  SCOPED_MUTEX_LOCK(lock, _mutex, this_ethread());
+  EThread *ethread = this_ethread();
+  if (ethread == nullptr) {
+    // During static destruction, EThreads are gone. Just delete directly.
+    _list.erase(plugin);
+    delete plugin;
+    return;
+  }
+
+  SCOPED_MUTEX_LOCK(lock, _mutex, ethread);
 
   _list.erase(plugin);
-  this_ethread()->schedule_imm(new DeleterContinuation<PluginDso>(plugin));
+  ethread->schedule_imm(new DeleterContinuation<PluginDso>(plugin));
 }
 
 /* check if need to reload the plugin DSO