Fix crash after re-reading an empty resolv.conf file.

If dnsmasq re-reads a resolv file, and it's empty, it will
retry after a delay. In the meantime, the old servers from the
resolv file have been deleted, but the servers_array doesn't
get updated, leading to dangling pointers and crashes.

Thanks to Brad Jorsch for finding and analysing this bug.

This problem was introduced in 2.86.

src/dnsmasq.c

@@ -1682,6 +1682,11 @@ static void poll_resolv(int force, int do_reload, time_t now)
 	}
       else 
 	{
+	  /* If we're delaying things, we don't call check_servers(), but 
+	     reload_servers() may have deleted some servers, rendering the server_array
+	     invalid, so just rebuild that here. Once reload_servers() succeeds,
+	     we call check_servers() above, which calls build_server_array itself. */
+	  build_server_array();
 	  latest->mtime = 0;
 	  if (!warned)
 	    {