Fix crash if audio gets disposed before content

Source/Engine/Audio/AudioBackend.h

@@ -14,7 +14,7 @@ class AudioBackend
     friend AudioBackend;
     friend class AudioService;
 
-private:
+public:
 
     static AudioBackend* Instance;
 

Source/Engine/Audio/AudioClip.cpp

@@ -413,6 +413,8 @@ Asset::LoadResult AudioClip::load()
         return LoadResult::MissingDataChunk;
 
     // Create single buffer
+    if (!AudioBackend::Instance)
+        return LoadResult::Failed;
     uint32 bufferId;
     AudioBackend::Buffer::Create(bufferId);
     Buffers[0] = bufferId;