NULL frame transmission in conference bridge is not clocked at the right interval

[bennylp]

When the port is not receiving signal from any other ports (e.g. when no other ports is transmitting to this port), the bridge periodically transmit NULL frame to the port to keep the port "alive" (for example, a stream port needs this heart-beat to periodically transmit silence frame to keep NAT binding alive).

This NULL frame should be sent to the port at the port's ptime rate. So if the port's ptime is greater than the bridge's ptime, the bridge needs to delay the NULL frame until it's the right time to do so.

This bug transmits NULL frame at the bridge's ptime interval and not the port's ptime interval.

[bennylp]

Fixed in r1839