osdc/Striper: test zero-length add_partial_result

If we add a partial result that is 0-length, we used to hit an assert in
buffer::list::splice().  Add a unit test to verify the fix.

Signed-off-by: Sage Weil <sage@inktank.com>
buffer: make 0-length splice() a no-op

This was causing a problem in the Striper, but fixing it here will avoid
corner cases all over the tree.  Note that we have to bail out before
the end-of-buffer check to avoid hitting that check when the bufferlist is
also empty.

Signed-off-by: Sage Weil <sage@inktank.com>