IAX2: Transfer Reject: Lock bridgecallno before touching it, refactor

1). When touching the bridgecallno, we need to lock it.

2). Remove magic number '0' and replace with TRANSFER_NONE.

3). Exit early if no bridgecallno.

4). Reduce indentation.

Reported by: alecdavis
Tested by: alecdavis
alecdavis (license 585)

Review https://reviewboard.asterisk.org/r/2613/



git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@391333 f38db490-d61c-443f-a65b-d21fe96a405b