No server buffers remain to edit, so don't switch buffers

[Dan Jacobson]

If we mistakenly hit server-edit (C-x #) when there are no server
buffers that remain to edit, we get a message "No server buffers
remain to edit", but we are switched to another buffer anyway.

P.S., an unrelated situation, probably not worth fixing as it would
break more general cases:
M-x shell
$ emacsclient bla
M-x b some_buffer
M-x b bla
C-x #
But now we should be back in the calling buffer, *shell*, but we aren't.