RTC: Fix both rtmp and rtc streams can be published at the same time.

[chundonglinlin]

When I publish rtmp firstly and then publish rtc, both streams can be published. I hope there was only one stream. My config as follows,

vhost __defaultVhost__ {
    rtc {
        enabled     on;
        # @see https://github.com/ossrs/srs/wiki/v4_CN_WebRTC#rtmp-to-rtc
        rtmp_to_rtc off;
        # @see https://github.com/ossrs/srs/wiki/v4_CN_WebRTC#rtc-to-rtmp
        rtc_to_rtmp off;
    }
}