simplify function __ast_request_and_dial() as follows:

simplify function __ast_request_and_dial() as follows:
- handle immediately failures in ast_request();
  This removes the need for checking 'chan' multiple times afterwards.
- handle immediately failures in ast_call(), by moving the one-line
  case at the top of the 'if' statement;
- use ast_strlen_zero in several places instead of expanding it inline;
- make outstate always a valid pointer;
On passing mark an unclear statement and replace a magic number
with sizeof(tmp).

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@20511 65c4cc65-6c06-0410-ace0-fbb531ad65f3