Do not dial digits when none were specified for sig_pri based calls

(closes issue #15524)
Reported by: elguero
Patches:
      pri-sig-no-dest-set.patch uploaded by elguero (license 37)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@207950 65c4cc65-6c06-0410-ace0-fbb531ad65f3