element: NULL the lists of contexts in dispose()

If dispose() is called more than once, we may double unref the list of
GstContext's.

Part-of: <gstreamer/gstreamer!876>
34 jobs for !876 with backport-mr-875-into-1.18-dispose-context-null in 25 minutes and 53 seconds (queued for 5 seconds)
merge request