Bug 13114: Prevent Shibboleth Patches from spamming logs

- The shibboleth patch introduced an undefined message into the error
  logs, when shiboleth is disabled.

Testplan

1. Ensure shibboleth is disabled.
2. Refresh any opac page
3. See 'Use of uninitialized value $ENV{"REMOTE_USER"} in string ne at
/home/koha/kohaclone/C4/Auth.pm line 711.' popup in the opac-error.log
4. Apply patch
5. Refresh opac page
6. Error should no longer appear

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Works as described.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>

C4/Auth.pm

@@ -708,7 +708,7 @@ sub checkauth {
     # Finally, after those tests, we can assume (although if it would be better with
     # a syspref) that if we get a REMOTE_USER, that's from basic authentication,
     # and we can affect it to $userid.
-    if ( !$shib and $ENV{'REMOTE_USER'} ne '' and $userid = $ENV{'REMOTE_USER'} ) {
+    if ( !$shib and defined($ENV{'REMOTE_USER'}) and $ENV{'REMOTE_USER'} ne '' and $userid = $ENV{'REMOTE_USER'} ) {
 
         # Using Basic Authentication, no cookies required
         $cookie = $query->cookie(