fixes ast_transfer stall until hangup if called with a channel that doesn't support transfers

ast_transfer sets res to 0 if there is no technology transfer function,
but then tests for it to be negative before deciding to do an early exit.
As a result, it will will wait for an AST_CONTROL_TRANSFER message that
will never come.

(closes issue #16424)
Reported by: davidw
Patches:
      Issue_16424_trunk_234134.patch uploaded by davidw (license 780)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@238492 65c4cc65-6c06-0410-ace0-fbb531ad65f3