Fix AAsn to not die horribly when non-reference is returned from some function

git-svn-id: https://svn.code.sf.net/p/xcat/code/xcat-core/branches/2.8@15837 8638fb3e-16cb-4fca-ae20-7b5d299a9bcd

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

@@ -1063,6 +1063,11 @@ sub setup_TFTP
 
     # read DB for nodeinfo
     my $retdata = xCAT::ServiceNodeUtils->readSNInfo($nodename);
+    if (not ref $retdata)
+    {    # error
+        xCAT::MsgUtils->message("S", " Error reading service node arch.");
+        return 1;
+    }
     $master = $retdata->{'master'};
     $os     = $retdata->{'os'};
     $arch   = $retdata->{'arch'};