The -1 return value from incomplete or improper

headers for the SipNotify manager command was
causing the current manager session to become
disconnected. Change the return value to 0 for
these cases.

Also change a test for a NULL pointer to be
ast_strlen_zero instead.

(closes issue #13351)
Reported by: Laureano
Patches:
      sipnotify_action_fix.patch uploaded by Laureano (license 265)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@139563 65c4cc65-6c06-0410-ace0-fbb531ad65f3