(closes issue #13489)

(closes issue #13489)
Reported by: DougUDI
Tested by: murf

(closes issue #13490)
Reported by: seanbright
Tested by: murf

(closes issue #13467)
Reported by: edantie
Tested by: murf, edantie, DougUDI

This crash happens because we are unsafely handling old pointers.
The channel whose cdr is being handled, has been hung up and
destroyed already. I reorganized the code a bit, and tried not
to lose the fork-cdr-chain concepts of the previous code.
I now verify that the 'previous' channel (the channel we
had when the bridge was started), still exists, by looking it up
by name in the channel list. I also do not try to reset the
CDR's of channels involved in bridges.

Testing shows it solves the crash problem, and should not
negatively impact previous fixes involving CDR's generated
during/after blind transfers. (The reason we need to reset
the CDR's on the "beginning" channels in the first place).

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@144066 65c4cc65-6c06-0410-ace0-fbb531ad65f3