Bug 368147: Upgrade from 2.16 and earlier does not work with DBD::mysql 4.0

Patch By Max Kanat-Alexander <mkanat@bugzilla.org> r=LpSolit, a=mkanat

webtools/bugzilla/Bugzilla/Install/DB.pm

@@ -1381,7 +1381,8 @@ sub _convert_groups_system_from_groupset {
         # so all unique keys are removed first and then added back in
         $dbh->bz_drop_index('groups', 'groups_bit_idx');
         $dbh->bz_drop_index('groups', 'groups_name_idx');
-        if ($dbh->primary_key(undef, undef, 'groups')) {
+        my @primary_key = $dbh->primary_key(undef, undef, 'groups');
+        if (@primary_key) {
             $dbh->do("ALTER TABLE groups DROP PRIMARY KEY");
         }