sd-event: unpoison memory returned by epoll_pwait2()

Our fuzzer CI recently got bumped to Ubuntu 24.04 with
glibc >= 2.35. Apparently msan is not happy with the new
epoll_pwait2(), hence explicitly mark the memory region
as initialized.

Fixes #40455
Alternative to #40463

src/libsystemd/sd-event/sd-event.c

@@ -4618,6 +4618,7 @@ static int process_epoll(sd_event *e, usec_t timeout, int64_t threshold, int64_t
                         return r;
 
                 m = (size_t) r;
+                msan_unpoison(e->event_queue, m * sizeof(struct epoll_event));
 
                 if (m < n_event_max)
                         break;