bridge_basic: don't cache xferfailsound during an attended transfer

The xferfailsound was read from the channel at the beginning of the transfer,
and that value is "cached" for the duration of the transfer. Therefore, changing
the xferfailsound on the channel using the FEATURE() dialplan function does
nothing once the transfer is under way.

This makes it so the transfer code instead gets the xferfailsound configuration
options from the channel when it is actually going to be used.

This patch also fixes a potential memory leak of the props object as well as
making sure the condition variable gets initialized before being destroyed.

ASTERISK-25696 #close

Change-Id: Ic726b0f54ef588bd9c9c67f4b0e4d787934f85e4