Bug 41904: "Use of uninitialized value..." warning in del_message.pl

The warning
[WARN] Use of uninitialized value in string eq at
/kohadevbox/koha/circ/del_message.pl line 54.
appears in plack-intranet-error.log

Test plan:
==========
1. Observe plack-intranet-error.log
2. Search for any patron and go to "Check out" tab.
3. Add message for patron. Added message is shown in "Message" area
   with "Edit" and "Delete" links.
4. Click "Delete" and confirm modal.
5. Warning appears in plack-intranet-error.log
6. Apply the patch; restart_all
7. Repeat 3-4. No warning appears.

Sponsored-by: Ignatianum University in Cracow

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Laura Escamilla <laura.escamilla@bywatersolutions.com>
(cherry picked from commit 42908f8c68)
Signed-off-by: Baptiste Wojtkowski <baptiste.wojtkowski@biblibre.com>

circ/del_message.pl

@@ -50,7 +50,7 @@ if ( $message
 
 $message->delete if $message && $op eq 'cud-delete';
 
-if ( $input->param('from') eq  "moremember" ) {
+if ( $input->param('from') && $input->param('from') eq "moremember" ) {
     print $input->redirect(
         "/cgi-bin/koha/members/moremember.pl?borrowernumber=$borrowernumber");
 } else {