Xmux: Value of 0 is treated the same as no value (#3835)

It turns out that some panels like to set `"xmux": {"maxConnections": 0,
"maxConcurrency": 0}`, and of course that fails now. To make their job
easier, let's treat `0` the same as not setting the parameter.

Again, I don't like that xray's defaults are hardcoded all over the
place. I would have liked a different default value for Xmux in a future
version, but it actually can't be done in practice because everybody
just copypastes the defaults from the docs into their own sourcecode
(and sometimes changes them silently to their own idea of a good
default)

infra/conf/transport_internet.go

@@ -263,7 +263,7 @@ func (c *SplitHTTPConfig) Build() (proto.Message, error) {
 		c.Host = c.Headers["Host"]
 	}
 
-	if c.Xmux.MaxConnections != nil && c.Xmux.MaxConcurrency != nil {
+	if c.Xmux.MaxConnections != nil && c.Xmux.MaxConnections.To > 0 && c.Xmux.MaxConcurrency != nil && c.Xmux.MaxConcurrency.To > 0 {
 		return nil, errors.New("maxConnections cannot be specified together with maxConcurrency")
 	}