src/utils_llist.c: Improved robustness against NULL pointers.

src/utils_llist.c: Improved robustness against NULL pointers.

All functions searching for some list entry now return NULL if the given
list equals NULL. Imho, that's the expected behavior. Also, it removes the
need to check the lists from the calling function while that function
usually checks the returned list entry anyway.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>