[UI] Prevent a crash in the cover resource

src/ui/resource/CoverResource.cpp

@@ -41,9 +41,11 @@ CoverResource:: ~CoverResource()
 	beingDeleted();
 }
 
-const Image::Image&
+Image::Image
 CoverResource::getDefaultCover(std::size_t size)
 {
+	std::unique_lock<std::mutex> lock(_mutex);
+
 	auto itCover = _defaultCovers.find(size);
 	if (itCover == _defaultCovers.end())
 	{
@@ -163,7 +165,6 @@ CoverResource::handleRequest(const Wt::Http::Request& request, Wt::Http::Respons
 
 		// If no cover found, just send default one
 		putCover(response, getDefaultCover(size));
-
 	}
 	catch (std::invalid_argument& e)
 	{

src/ui/resource/CoverResource.hpp

@@ -47,7 +47,7 @@ class CoverResource : public Wt::WResource
 
 	private:
 
-		const Image::Image&		getDefaultCover(std::size_t size);
+		Image::Image			getDefaultCover(std::size_t size);
 		void				putCover(Wt::Http::Response& response, Image::Image image);
 
 		std::mutex			_mutex;