modify the getipaddr so that it would not warning when can't find the according hostname of a IP.

git-svn-id: https://svn.code.sf.net/p/xcat/code/xcat-core/trunk@8400 8638fb3e-16cb-4fca-ae20-7b5d299a9bcd

perl-xCAT/xCAT/NetworkUtils.pm

@@ -151,7 +151,9 @@ sub gethostname()
                 return undef;
             }
             my $hostname = gethostbyaddr(inet_aton($iporhost), AF_INET);
-            $hostname =~ s/\..*//; #short hostname
+            if ( $hostname ) {            
+                $hostname =~ s/\..*//; #short hostname
+            }
             return $hostname;
         }
      }