inter: Unref bus after usage

gst_pipeline_get_bus() returns a reference to a bus that needs to
be unreferenced after usage.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=734525
wildmidi: Avoid leaking copy of caps object

gst_pad_get_pad_template_caps() returns a reference which is unreferenced, so
creating a copy using gst_caps_copy() results in a reference leak.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=734529