Fix warning when gethostbyaddr did not return anything

[egore]

I get the following error lots of times:
Use of uninitialized value $host in substitution (s///) at ./mytop line 958.

This happens when gethostbyaddr returns null. In that case we should show the IP instead:

From 602d063dbac4ae94b2d2273ceef9fbf01f9ca1d9 Mon Sep 17 00:00:00 2001
From: Christoph Brill <[email protected]>
Date: Tue, 27 Aug 2013 16:11:13 +0200
Subject: [PATCH] If gethostbyaddr return nothing, keep using the IP

---
 mytop | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mytop b/mytop
index fba27fd..8d6bf42 100755
--- a/mytop
+++ b/mytop
@@ -955,8 +955,11 @@ sub GetData()
         {
             $thread->{Host} =~ s/:\d+$//;
             my $host = gethostbyaddr(inet_aton($thread->{Host}), AF_INET);
-            $host =~ s/^([^.]+).*/$1/;
-            $thread->{Host} = $host;
+            if ($host)
+            {
+                $host =~ s/^([^.]+).*/$1/;
+                $thread->{Host} = $host;
+            }
         }

         ## Fix possible undefs
-- 
1.8.4