MINOR: wdt: always clear sigev_value to make valgrind happy

In issue #471 it was reported that valgrind sometimes complains about
timer_create() being called with uninitialized bytes. These are in fact
the bits from sigev_value.sival_ptr that are not part of sival_int that
are tagged as such, as valgrind has no way to know we're using the int
instead of the ptr in the union. It's cheap to initialize the field so
let's do it.

src/wdt.c

@@ -129,7 +129,7 @@ void wdt_handler(int sig, siginfo_t *si, void *arg)
 
 int init_wdt_per_thread()
 {
-	struct sigevent sev;
+	struct sigevent sev = { };
 	sigset_t set;
 
 	/* unblock the WDTSIG signal we intend to use */