Fix double free crash during encrypted backups

Calling flush_libtar_buffer would inadvertantly set buffer_status
when no buffer is currently allocated. Later when we called
free_libtar_buffer, this would attempt to free the write_buffer
and cause a crash. This patch checks the value of buffer_status in
flush_libtar_buffer to prevent us from freeing the buffer when it
is not necessary.

Change-Id: Ic5b462fe5881be6e9c6fcc355b369bc90477f737