islet_login.sh: test for config after timeout

If TMOUT is reached because a user has not selected
an item in the menu then script the exec's islet_login
and causes a source failure shown below. If that happens
we should just exit the program to be safe.

Choose a number to load the configuration:

1) argus.conf	     4) jenkins.conf	  7) tcpdump.conf
2) brolive.conf	     5) netsniff-ng.conf
3) ids.conf	     6) sniffer.conf
[Choice]:
/opt/islet/bin/islet_login: line 26: source: /etc/islet/: is a directory
Are you a new or existing user? [new/existing]:
...
Connection to 127.0.0.1 closed.