bridge.c: Fixed race condition during attended transfer

During an attended transfer a thread is started that handles imparting the
bridge channel. From the start of the thread to when the bridge channel is
ready exists a gap that can potentially cause problems (for instance, the
channel being swapped is hung up before the replacement channel enters the
bridge thus stopping the transfer). This patch adds a condition that waits
for the impart thread to get to a point of acceptable readiness before
allowing the initiating thread to continue.

ASTERISK-24782
Reported by: John Bigelow

Change-Id: I08fe33a2560da924e676df55b181e46fca604577