End stuck DTMF if AST_SOFTHANGUP_ASYNCGOTO because it isn't a real hangup.

It doesn't hurt to check AST_SOFTHANGUP_UNBRIDGE either, but it should not
be set outside of a bridge.

(issue ASTERISK-20492)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@381466 f38db490-d61c-443f-a65b-d21fe96a405b