Don't send presencestate information if the state is invalid

Don't send presencestate information if the state is invalid

Previously, presencestate information was sent whenever the state was not
NOT_SET. When r381594 actually returned INVALID presence state in all the
places it was supposed to, it caused chan_sip to start adding presence
state information to NOTIFY requests that it previously would not have
added. chan_sip shouldn't be adding presence state information when the
provider is in an invalid state; users can't set the state to invalid and
an invalid state always implies that the provider is in an error condition.

(issue AST-1084)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@381613 65c4cc65-6c06-0410-ace0-fbb531ad65f3