Fix erroneous hangup extension execution

ast_spawn_extension behaves differently from 1.4 in that hangups and extensions
that do not exist do not return an error, whereas in 1.6 it does. This is now 
taken into account so that the AST_FLAG_BRIDGE_HANGUP_RUN flag gets set
properly.

(closes issue #16106)
Reported by: ajohnson
Tested by: ajohnson


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@231095 65c4cc65-6c06-0410-ace0-fbb531ad65f3