Sometimes after the M-x slime command, SLIME halt in the *inferior-lisp* buffer while
CLISP server is already up. Then I must slime-abort-connection and slime-connect manually.

The cause of halt is #'read encountered a zero-length slime-swank-port-file
in the slime-read-swank-port function in slime.el. The content of the port-file
has not been sync to the disk after CLISP server create the file, but file-exists-p is T.
To avoid this problem, we can add a file-length-gt-zero-p or sleep-for a while
after the file-exists-p before the call.