manager: Fix crash when appending a manager channel variable

In r399887, a minor performance improvement was introduced by not allocating
the manager variable struct if it wasn't used. Unfortunately, when directly
accessing an ast_channel struct, manager assumed that the struct was always
allocated. Since this was no longer the case, things got a bit crashy.

This fixes that problem by simply bypassing appending variables if the manager
channel variable struct isn't there.
........

Merged revisions 400058 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@400059 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/manager.c

@@ -5934,6 +5934,10 @@ static void append_channel_vars(struct ast_str **pbuf, struct ast_channel *chan)
 
 	vars = ast_channel_get_manager_vars(chan);
 
+	if (!vars) {
+		return;
+	}
+
 	AST_LIST_TRAVERSE(vars, var, entries) {
 		ast_str_append(pbuf, 0, "ChanVariable(%s): %s=%s\r\n", ast_channel_name(chan), var->name, var->value);
 	}