ERROR:owr_transport_agent.c:917:remove_existing_send_source_and_payload: assertion failed: (sinkpad)

On iOS Native,

```
cerbero version 2015/12/4
openssl: Bump to 1.0.2e
git:4e8ec2994bb3298db37c3f84356260e86ea3e07a
```

when the streaming didnt connected and call below api from reset() will crash every time.

```
owr_media_session_set_send_source(media_session, NULL);
```

but when it's connected to other whatever audio or video are all fine.

crash log call stack below:
https://gist.github.com/xelven/e7387e494b79a0b8090b

Start with owr_media_session_set_send_source(media_session, NULL) in the reset() path on iOS Native, then follow the crash stack to remove_existing_send_source_and_payload at owr_transport_agent.c:917. Reproduce while streaming is disconnected and compare with the connected audio/video cases. Done means resetting with a NULL send source no longer triggers the sinkpad assertion.

Written by the indexing model from the issue text.