Animations: Avoid accessing invalid memory if stop() triggers deletion

Return immediately if calling stop() results in the current object being
deleted.

Task-number: QTBUG-31247
Change-Id: I684a858bfb4cc53f415daa8cc1c4df96067daa1c
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Make sure tryCompare value argument is not undefined

It happens often that people convert a
compare(foo.bar, 3)
to a
tryCompare(foo.bar, 3)
and unfortunately that succeeds but doesn't do what they expected

This makes tryCompare fail if no third argument is given

Task-number: QTBUG-31427

Change-Id: I0c9618dae9aad4be55aa35c3e2dcf3535728beaa
Reviewed-by: Alan Alpert <aalpert@blackberry.com>