res_pjsip_session: Ensure reused streams have correct bundle group

When a bundled stream is removed, its bundle_group is reset to -1.
If that stream is later reused, the bundle parameters on session
media need to be reset correctly it could mistakenly be rebundled
with a stream that was removed and never reused.  Since the removed
stream has no rtp instance, a crash will result.

Change-Id: Ie2b792220f9291587ab5f9fd123145559dba96d7