obs-ffmpeg, obs-outputs: Check return of obs_encoder_get_extra_data

A race condition can occur in obs-outputs where the send_thread is in
the process of (re)connecting but the encoder was shut down in the
meantime. This causes the expected header data to be garbage, resulting
in a crash.
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f