Patch to prevent stopping the active generator when it is not the silence

Patch to prevent stopping the active generator when it is not the silence
generator.

This patch introduces an internal helper function to safely check whether the
current generator is the one that is expected before deactivating it. The
current externally accessible ast_channel_stop_generator() function has been
modified to be implemented in terms of the new function.

(closes issue ASTERISK-19918)
Reported by: Eduardo Abad
........

Merged revisions 376217 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@376291 65c4cc65-6c06-0410-ace0-fbb531ad65f3