workaround possibly false positive uses of memory after it is freed

workaround possibly false positive uses of memory after it is freed

scan-build from clang-9 has reported following:

 libubox/list.h:83:22: warning: Use of memory after it is freed
        entry->next->prev = entry->prev;
                            ^~~~~~~~~~~

 ubusd_event.c:42:3: warning: Use of memory after it is freed
                ubusd_delete_event_source(ev);
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Which might be a false positives, but in order to make the code pass the
static analyzer checks, rewrite the while loops on lists with the safe
list iterator.

Signed-off-by: Petr Štetiar <ynezz@true.cz>