Don't crash if the current track source encounters a record with no id

This *shouldn't* normally happen, but it's not worth crashing over.

src/audio/track_queue.cpp

@@ -100,6 +100,7 @@ auto TrackQueue::AddNext(std::shared_ptr<playlist::ISource> src) -> void {
 
 auto TrackQueue::IncludeNext(std::shared_ptr<playlist::IResetableSource> src)
     -> void {
+  assert(src.get() != nullptr);
   const std::lock_guard<std::mutex> lock(mutex_);
   enqueued_.push_front(src);
 
@@ -128,6 +129,7 @@ auto TrackQueue::AddLast(std::shared_ptr<playlist::ISource> src) -> void {
 
 auto TrackQueue::IncludeLast(std::shared_ptr<playlist::IResetableSource> src)
     -> void {
+  assert(src.get() != nullptr);
   const std::lock_guard<std::mutex> lock(mutex_);
   enqueued_.push_back(src);
 

src/playlist/source.cpp

@@ -139,9 +139,12 @@ auto IndexRecordSource::Peek(std::size_t n, std::vector<database::TrackId>* out)
       working_item = 0;
     }
 
-    out->push_back(working_page->values().at(working_item)->track().value());
-    n--;
-    items_added++;
+    auto record = working_page->values().at(working_item);
+    if (record->track()) {
+      out->push_back(record->track().value());
+      n--;
+      items_added++;
+    }
     working_item++;
   }