core/clist: add cppcheck-suppress nullPointer

This was found by cppcheck 1.6x used in Travis-CI, however this
seems to be a false positive, as it is not found by newer versions
of cppcheck.

core/clist.c

@@ -78,6 +78,8 @@ clist_node_t *_clist_sort(clist_node_t *list, clist_cmp_func_t cmp)
             for (i = 0; i < insize; i++) {
                 psize++;
                 q = (q->next == oldhead) ? NULL : q->next;
+                /* cppcheck-suppress nullPointer
+                 * (reason: possible bug in cppcheck 1.6x) */
                 if (!q) {
                     break;
                 }