Moved the unused RageSoundReader_Silence from RageSound.cpp to RageSoundReader.h and use it in AutoKeysounds.cpp.

Hopefully this should fix Bug 0000500, but I don't know, can't reproduce.

src/AutoKeysounds.cpp

@@ -19,6 +19,7 @@
 #include "AutoKeysounds.h"
 #include "GameState.h"
 #include "Song.h"
+#include "RageSoundReader.h"
 #include "RageSoundReader_Chain.h"
 #include "RageSoundReader_ChannelSplit.h"
 #include "RageSoundReader_Extend.h"
@@ -276,6 +277,8 @@ void AutoKeysounds::FinishLoading()
 		else
 		{
 			delete pChain;
+			RageSoundReader_Silence *pSilence = new RageSoundReader_Silence;
+			m_pSharedSound = pSilence;
 		}
 	}
 	ASSERT_M( m_pSharedSound, ssprintf("No keysounds were loaded for the song %s!", pSong->m_sMainTitle.c_str() ));