buffer: Merge branch 'evbuffer-empty-chains-fixes-v4'

This fixes bug with empty-chains and
evbuffer_add_buffer()/evbuffer_remove_buffer().

* evbuffer-empty-chains-fixes-v4:
  buffer: evbuffer_add_buffer(): clean empty chains from destination buffer
  test/buffer: evbuffer_add_buffer() with empty chains
  test/buffer: evbuffer_remove_buffer() with empty chains (prepend)
  test/buffer: evbuffer_remove_buffer() with empty chains (evbuffer_add_buffer())

Reported-by: Eduardo Panisset <eduardo@anchorfree.com>