Fix deadlock when using dummy channels.

Dummy channels created by ast_dummy_channel_alloc() should be destoyed by
ast_channel_unref().  Using ast_channel_release() needlessly grabs the
channel container lock and can cause a deadlock as a result.

* Analyzed use of ast_dummy_channel_alloc() and made use
ast_channel_unref() when done with the dummy channel.  (Primary reason for
the reported deadlock.)

* Made app_dial.c:dial_exec_full() not call ast_call() holding any channel
locks.  Chan_local could not perform deadlock avoidance correctly.
(Potential deadlock exposed by this issue.  Secondary reason for the
reported deadlock since the held lock was part of the deadlock chain.)

* Fixed some uses of ast_dummy_channel_alloc() not checking the returned
channel pointer for failure.

* Fixed some potential chan=NULL pointer usage in func_odbc.c.  Protected
by testing the bogus_chan value.

* Fixed needlessly clearing a 1024 char auto array when setting the first
char to zero is enough in manager.c:action_getvar().

(closes issue ASTERISK-18613)
Reported by: Thomas Arimont
Patches:
      jira_asterisk_18613_v1.8.patch (license #5621) patch uploaded by rmudgett
Tested by: Thomas Arimont


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@337973 65c4cc65-6c06-0410-ace0-fbb531ad65f3

cdr/cdr_syslog.c

@@ -116,7 +116,7 @@ static int syslog_log(struct ast_cdr *cdr)
 
 	AST_RWLIST_UNLOCK(&sinks);
 
-	ast_channel_release(dummy);
+	ast_channel_unref(dummy);
 
 	return 0;
 }