app_queue: Prevent crash when a call is forwarded to an invalid location

When a call forward attempt is made from a Queue member, the current
code will hang up the forwarding channel in an off-nominal condition
prior to raising the Stasis events informing the rest of Asterisk that
the call was forwarded. This will result in a slew of dreaded FRACKs,
most likely leading to a crash.

This patch modifies the code such that we don't hang up the forwarding
channel even in an off-nominal condition until we've safely raised the
Stasis messages.

ASTERISK-25797 #close

Change-Id: Ife5abed351691fd79105321636eaa8ea8dcdba38