When ENABLE_DEBUG is 1, `out` is dereferenced unconditionally, but `_parse_options` in `gnrc_rpl_control_messages.c` calls it with NULL. Clarified `out` must not NULL and fixed `_parse_options`.
When ENABLE_DEBUG is 1, `out` is dereferenced unconditionally, but `_parse_options` in `gnrc_rpl_control_messages.c` calls it with NULL. Clarified `out` must not NULL and fixed `_parse_options`.