flvmux accesses the GstElement sinkpads list unsafely.

I just fixed one case that showed up in the CI of flvmux accessing the GstElement sinkpads list directly without holding the GST_OBJECT_LOCK(), but it does it in more places. It should either hold the lock in those places too, or use gst_element_iterate_sink_pads() or gst_element_foreach_sink_pad()