fix: fail nodeset when MASTER_IP cannot be resolved

[viniciusferrao]

Summary

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.

The fix applies the same $tmplerr + return pattern already used elsewhere in the same function (e.g., the unless ($master) check two lines above).

What changed

# Before: silently continues with empty MASTER_IP
my $ipaddr = xCAT::NetworkUtils->getipaddr($master);
if ($ipaddr) {
    $ENV{MASTER_IP} = "$ipaddr";
}

# After: fails nodeset with a clear error
my $ipaddr = xCAT::NetworkUtils->getipaddr($master);
unless ($ipaddr) {
    $tmplerr = "Unable to resolve master \"$master\" to an IP address for $node";
    return;
}
$ENV{MASTER_IP} = "$ipaddr";

Test plan

• Follows existing $tmplerr pattern (4 other instances in same function)
• Happy path unchanged (successful resolution sets MASTER_IP as before)
• git diff --check clean
• Error path now returns descriptive message to nodeset caller

Fixes xcat2#7544