libs/gst/controller/gstcontroller.c: Don't g_return_val_if_fail() on timed values with invalid timestamps inside a cr...

Original commit message from CVS:
* libs/gst/controller/gstcontroller.c: (gst_controller_new_valist),
(gst_controller_set_from_list):
Don't g_return_val_if_fail() on timed values with invalid timestamps
inside a critical section without unlocking the mutex. Spotted by
René Stadler. (#357617)
Also, fix up refcounting properly: when returning an existing
controller, we should increase the reference only once and not
once per property and when trying to control a property again
we should also increase the refcount.