AST-2012-009: Fix crash in chan_skinny due to Key Pad Button Message handling

AST-2012-008 (r367844) fixed a denial of service attack exploitable in the
Skinny channel driver that occurred when certain messages are sent after a
previously registered station sends an Off Hook message.  Unresolved in that
patch is an issue in the Asterisk 10 releases, wherein, if a Station Key
Pad Button Message is processed after an Off Hook message, the channel driver
will inappropriately dereference a NULL pointer.

This patch fixes those places where the message handling or the channel
callback functions would attempt to dereference the line's pointer to the
device.

(issue ASTERISK-19905)
Reported by: Christoph Hebeisen
Tested by: mjordan, Christoph Hebeisen
Patches:
  AST-2012-009-10.diff uploaded by mjordan (license 6283)
........

Merged revisions 368947 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@368948 65c4cc65-6c06-0410-ace0-fbb531ad65f3