Resolve a crash related to a T.38 reinvite race condition.

Resolve a crash related to a T.38 reinvite race condition.

This change resolves a crash observed locally during some T.38 testing.
A call was set up using a call file, and when the T.38 reinvite came in,
the channel state was still AST_STATE_DOWN.  The reason is explained by
a comment in the code that previously lived in the handling of
AST_STATE_RINGING.  This change modifies the logic to handle the same
race condition for any channel state that is not UP.

(closes ABE-1895)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@203115 65c4cc65-6c06-0410-ace0-fbb531ad65f3