By putting uridecodebin into a bin with a ghostpad. Without this,
nlesource tries to get a srcpad too early (before uridecodebin added
one) and everything fails miserably.

This has to be fixed properly in nlesource at some point, by properly
handling dynamically added pads. Currently they can only work if they
are added in states <= READY, which is not the usual case.

https://bugzilla.gnome.org/show_bug.cgi?id=771843