rollupdate bug -- fixed undefined ref when using nodegroup_mutex and no mutex exists

git-svn-id: https://svn.code.sf.net/p/xcat/code/xcat-core/trunk@8620 8638fb3e-16cb-4fca-ae20-7b5d299a9bcd

xCAT-server/lib/xcat/plugins/rollupdate.pm

@@ -1607,7 +1607,7 @@ mxnode_loop:  foreach my $mxnode ( xCAT::NodeRange::noderange($mxnodegroup) ) {
               } # end mxnode_loop
               if ($mxpi == 1) {
                  # only one updategroup in this mutex, not valid -- ignore it
-                 undef $::MUTEX[$mxindex];
+                 delete $::MUTEX[$mxindex];
               } elsif ( $mxpi > 1 ) {
                  $::MUTEX_COUNT[$mxindex] = $mx_count;
                  $mxindex++;