res_pjsip_session: Reset pending_media_state->read_callbacks

res_pjsip_session: Reset pending_media_state->read_callbacks

In handle_negotiated_sdp the pending_media_state->read_callbacks must be
reset before they are added in the SDP handlers in
handle_negotiated_sdp_session_media. Otherwise, old callbacks for
removed streams and file descriptors could be added to the channel and
Asterisk would poll on non-existing file descriptors.

Resolves: #611
(cherry picked from commit f626af8e13f00deeed0a0220b9af9423508acc0c)