net.server_epoll: Prevent loop attempting to send last data after close

If the connection is closed by the peer, any buffered data is given a
last chance to be sent (see f27b9319e0da). If the connection is Really
closed, no attempt to write will occur, instead epoll will raise the
error flag and :onreadable() will be invoked again, where it will try to
:close() again for the same reason, thus looping until the connection
somehow gets destroyed.

By clearing the _connected flag, the second time it passes :onreadable()
it should go directly to :destroy(), breaking the loop.

Thanks Link Mauve for reporting

net/server_epoll.lua

@@ -577,6 +577,7 @@ interface.send = interface.write;
 -- Close, possibly after writing is done
 function interface:close()
 	if self._connected and self.writebuffer and (self.writebuffer[1] or type(self.writebuffer) == "string") then
+		self._connected = false;
 		self:set(false, true); -- Flush final buffer contents
 		self:setreadtimeout(false);
 		self:setwritetimeout();