pad: Don't leak user_data in gst_pad_start_task
When the task already exists, we forgot to free the passed user_data
.
This wasn't an issue for most C code, which doesn't pass a
GDestroyNotify
, but bindings such as gstreamer-rs do!
That said, allocating a trampoline in gstreamer-rs just for it to get
thrown away again is awkward. Maybe we need a gst_pad_resume_task
?