gst: Fix floating reference inconsistencies in error cases

gst: Fix floating reference inconsistencies in error cases

If a function takes a floating reference and sinks it, it should also do
that in error cases. I.e. call ref_sink() followed by unref().

Otherwise the reference counting behaviour of the function will be
different between the good and the error case, and simply inconsistent.

https://bugzilla.gnome.org/show_bug.cgi?id=747990