Fix: "Selecting deleted buffer"

When wlf:set-buffer get a killed buffer, "Selecting deleted buffer"
error is raised.  This could prevent raising the backtrace window
because wlf may hit the same error again.
Treat the case when BUF is a string in wlf:set-buffer
Merge pull request #4 from tkf/fix-set-buffer

Fix: "Selecting deleted buffer"