Function `strip_effect_get_new_inputs` contained selection parsing and
error handling. It also returned values by setting 3 pointer addresses.
Split function to selection parsing which returns strips in container
and validation function, which returns error string if selection is not
valid. If selection is valid, empty string is returned
Pull Request: https://projects.blender.org/blender/blender/pulls/136474
972fda1a7d