Skinny - Fix deadlock when pickup of no call.

Locking issues in skinny when picking up a call that doesn't exist. Cleaned 
up sub locking by fully removing and using the chan lock instead. Also
changed ast_call_pickup to check whether chan was masq'd.

(closes issue ASTERISK-23249)
Reported by: wedhorn
Tested by: snuffy, myself
Patches: 
    skinny-locking01.diff uploaded by wedhorn (license 5019)
........

Merged revisions 407197 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@407198 65c4cc65-6c06-0410-ace0-fbb531ad65f3