fix: fail nodeset when MASTER_IP cannot be resolved

Template.pm silently continued rendering kickstart templates when
getipaddr() failed to resolve the master hostname, producing
kickstarts with an empty MASTER_IP. Nodes would install successfully
but fail on first reboot when post.xcat and xcatinstallpost tried
to contact the master, timing out after 90 retries with:

    the network between the node and  is not ready

Postage.pm (mypostscript generation) already checks for this and
returns a clear error. Apply the same pattern in Template.pm so
nodeset fails immediately with a descriptive message instead of
producing a broken kickstart.

Fixes #7544

xCAT-server/lib/perl/xCAT/Template.pm

@@ -123,9 +123,11 @@ sub subvars {
 
     $ENV{XCATMASTER} = $master;
     my $ipaddr = xCAT::NetworkUtils->getipaddr($master);
-    if ($ipaddr) {
-        $ENV{MASTER_IP} = "$ipaddr";
+    unless ($ipaddr) {
+        $tmplerr = "Unable to resolve master \"$master\" to an IP address for $node";
+        return;
     }
+    $ENV{MASTER_IP} = "$ipaddr";
 
     my @nodestatus = xCAT::TableUtils->get_site_attribute("nodestatus");
     my $tmp        = $nodestatus[0];