Bug 21336: (QA follow-up) Add column_exists in db revision

As requested by Josef on comment70.

Test plan:
Run db revision twice. No warnings.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>

installer/data/mysql/atomicupdate/bug_21336a.perl

@@ -1,7 +1,7 @@
 $DBversion = 'XXX';  # will be replaced by the RM
 if( CheckVersion( $DBversion ) ) {
-    $dbh->do( "ALTER TABLE borrowers ADD COLUMN flgAnonymized tinyint DEFAULT 0" );
-    $dbh->do( "ALTER TABLE deletedborrowers ADD COLUMN flgAnonymized tinyint DEFAULT 0" );
+    $dbh->do( "ALTER TABLE borrowers ADD COLUMN flgAnonymized tinyint DEFAULT 0" ) if !column_exists('borrowers', 'flgAnonymized');
+    $dbh->do( "ALTER TABLE deletedborrowers ADD COLUMN flgAnonymized tinyint DEFAULT 0" ) if !column_exists('deletedborrowers', 'flgAnonymized');
     SetVersion( $DBversion );
     print "Upgrade to $DBversion done (Bug 21336 - Add field flgAnonymized)\n";
 }