fixup! Ref count usage of MFile, mmap only when in use

src/lavinmq/mfile.cr

@@ -198,7 +198,7 @@ class MFile < IO
     # raise IO::Error.new("not memory mapped") if @buffer.null?
     pos = @pos
     len = Math.min(slice.size, @size - pos)
-    slice.copy_from(buffer + pos, len)
+    slice.copy_from(@buffer + pos, len)
     @pos = pos + len
     len
   end