chan_local: Only block media frames when a generator is on both ends of a local channel.

The fix for ASTERISK-12292 was a bit too aggressive.  You could have
generators pointed at each other on local channels but need to get other
kinds of frames such as DTMF or CONNECTED_LINE frames accross.


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@414269 f38db490-d61c-443f-a65b-d21fe96a405b