fix a memory leak in device state

The callback handle_statechange (pbx.c) fails to release its data
pointer, leaking memory in the process.

Reported by: tzafrir
Patches:
      18735_pbx_free_callback.diff uploaded by tzafrir (license 46)

Review: https://reviewboard.asterisk.org/r/1110/



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@308371 65c4cc65-6c06-0410-ace0-fbb531ad65f3