dtlssrtpenc/dec: Should reset its state when going back to NULL

Submitted by Olivier Crête @ocrete

Link to original bug (#747217)

Description

Currently, setting the srtp-/srtcp- properties destroys the dtls element, but there is not way to restore it when changing states.

It should probably re-create the sub-elements when going from NULL->READY if none of those propertties are set. Probalby also makes sense to re-create it if they are all resets to NULL.