Dangerous use of Py_XDECREF in _trait_set_validate

There's a potentially dangerous use of `Py_XDECREF` in `_trait_set_validate`: https://github.com/enthought/traits/blob/69b1a4701e320d28ee783ce7e8e25df96dce648a/traits/ctraits.c#L4290

The chance of this being exploited by accident is fairly low, but nonzero.

The issue is that the `Py_XDECREF` call can cause arbitrary Python code to run, and then that Python code sees a trait object in an inconsistent state.

This is very similar to issue #844, and the fix would be similar, too. Writing a regression test may not be straightforward.