splitmuxsink: Fix GObject warnings on shutdown.
Commit 83e718 added a pad template to splitmux request pads, which means that GstElement now releases the pads on dispose, but after having removed all elements in the bin and unlinked them. Make sure we can handle cleanup in that case without throwing assertions. https://bugzilla.gnome.org/show_bug.cgi?id=773784
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