kdig: set closed descriptor variable to -1

Coverity scan #1019149

Change-Id: I3beb8efced13d2bf5a5316ae02835739ffa54263

src/utils/common/netio.c

@@ -431,6 +431,7 @@ void net_close(net_t *net)
 	}
 
 	close(net->sockfd);
+	net->sockfd = -1;
 }
 
 void net_clean(net_t *net)