Fix crash in run_actions when calling quit

(cherry picked from commit 6315f28)

libcaf_core/caf/action.hpp

@@ -79,6 +79,10 @@ class CAF_CORE_EXPORT action {
     pimpl_->dispose();
   }
 
+  void swap(action& other) noexcept {
+    pimpl_.swap(other.pimpl_);
+  }
+
   // -- conversion -------------------------------------------------------------
 
   /// Returns a smart pointer to the implementation.

libcaf_core/src/scheduled_actor.cpp

@@ -930,9 +930,15 @@ void scheduled_actor::deregister_stream(uint64_t stream_id) {
 
 void scheduled_actor::run_actions() {
   if (!actions_.empty()) {
+    // Note: if the first actions is null, it means that we are already running
+    // actions right now. This can happen if an action calls `quit`, which will
+    // call `run_actions` again.
+    if (!actions_.front())
+      return;
     // Note: can't use iterators here since actions may add to the vector.
     for (auto index = size_t{0}; index < actions_.size(); ++index) {
-      auto f = std::move(actions_[index]);
+      action f;
+      f.swap(actions_[index]);
       f.run();
     }
     actions_.clear();