Fix some bogus scheduler usage in chan_sip. This code used the return value

Fix some bogus scheduler usage in chan_sip.  This code used the return value
of a completely unrelated function to determine whether the scheduler should
be run or not.  This would have caused the scheduler to not run in cases where
it should have.  Also, leave a note about another scheduler issue that needs
to be addressed at some point.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@140060 65c4cc65-6c06-0410-ace0-fbb531ad65f3