res_pjsip_transport_websocket: Prevent transport from being destroyed before message...

res_pjsip_transport_websocket: Prevent transport from being destroyed before message finishes.

From the gdb information, ast_websocket_read reads a message successfully,
then transport_read is called in the serializer. During execution of pjsip_transport_down,
ws_session->stream->fd is closed; ast_websocket_read encounters an error and exits the while loop.
After executing transport_shutdown, the transport's reference count becomes 0, causing a crash when sending SIP messages.
This was due to pjsip_transport_dec_ref executing earlier than pjsip_rx_data_clone, leading to this issue.
In websocket_cb executeing pjsip_transport_add_ref, this we now ensure the transport is not destroyed while in the loop.

Resolves: asterisk#299
(cherry picked from commit 3e3c2c833dcb6838a952bd90cddbb748d1a64f01)