libs/gst/controller/: Don't crash if someone tries to set an interpolation mode that is invalid or that isn't support...

Original commit message from CVS:
* libs/gst/controller/gstcontroller.c:
(gst_controlled_property_set_interpolation_mode):
* libs/gst/controller/gstinterpolation.c:
Don't crash if someone tries to set an interpolation mode that
is invalid or that isn't supported yet. Fixes #422295.
* tests/check/libs/controller.c: (GST_START_TEST),
(gst_controller_suite):
Add a test case for the above.