Jan Schmidt authored Oct 30, 2020 and Tim-Philipp Müller committed Oct 31, 2020

The main context can disappear in gst_webrtc_bin_enqueue_task()
between checking the is_closed flag and enqueueing a source on the
main context. Protect the main context with the object lock instead
of the PC lock, and hold a ref briefly to make sure it stays alive.

Part-of: <!1746>