add_custom_command: Manage backtrace memory correctly (#14299)

Add an assignment operator to cmCustomCommand to copy the Backtrace
member pointee and avoid multiple-free on destruction.

Reported-by: Vitezslav Cizek <vcizek@suse.cz>
Merge topic 'custom-command-assignment' into next

17c841c add_custom_command: Manage backtrace memory correctly (#14299)