fixup! core/thread_flags_group: use portable implementation

Author: maribu

core/thread_flags_group.c

@@ -47,6 +47,11 @@ void thread_flags_group_set(thread_flags_group_t *group, thread_flags_t mask)
             pid_block = bitarithm_test_and_clear(pid_block, &pid_offs);
             kernel_pid_t target_pid = pid_base + pid_offs;
             thread_t *target = thread_get(target_pid);
+            if (!target) {
+                DEBUG("|  %02u |   n/a |   n/a (dead) |   n/a (dead) |\n",
+                      target_pid);
+                continue;
+            }
             target->flags |= mask;
             thread_status_t old_status = target->status;
             bool awoken = thread_flags_wake(target);