[core] Ensure that the non-null peersInvolved is used

This is cached because we know the version passed into
the method is not guaranteed to be non-null.

However, we do have a guarantee that it will be non-null
when the final block for a given piece is received, so
by the time all blocks are written to disk we will have
a non null entry in the cache.

src/MonoTorrent/MonoTorrent.Client/Modes/Mode.cs

@@ -364,6 +364,7 @@ async void WritePieceAsync (PieceMessage message, IList<IPeer> peersInvolved)
 
             // All blocks have been written for this piece have been written!
             BlocksWrittenPerPiece.Remove (message.PieceIndex);
+            peersInvolved = data.peersInvolved;
 
             // Hashcheck the piece as we now have all the blocks.
             byte[] hash;