bridging: Ensure successful T.38 negotation

When a T.38 happens immediatly after call establishment, the control
frame can be lost because the other leg is not yet in the bridge.

This patch detects this case an makes sure T.38 negotation happens
when the 2nd leg is being made compatible with the negotating
first leg

ASTERISK-26923 #close

Change-Id: If334125ee61ed63550d242fc9efe7987e37e1d94