Run custom-magic-reset in the customize buffer

Run custom-magic-reset in the customize buffer

If the user has navigated away from the customize buffer, then
clicking on a widget in the customize buffer applies changes in the
selected buffer rather than in the customize buffer.  Pass the
customize buffer to 'custom-magic-reset' to avoid this.

* lisp/cus-edit.el (custom-magic-reset): Add optional buffer argument,
apply changes in that buffer.
(custom-notify): Pass the buffer containing the widget to
'custom-magic-reset'.  (Bug#40788)