unix: squelch harmless valgrind warning

PR-URL: https://github.com/libuv/libuv/pull/2895
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

src/unix/signal.c

@@ -143,6 +143,8 @@ static void uv__signal_block_and_lock(sigset_t* saved_sigmask) {
   if (sigfillset(&new_mask))
     abort();
 
+  /* to shut up valgrind */
+  sigemptyset(saved_sigmask);
   if (pthread_sigmask(SIG_SETMASK, &new_mask, saved_sigmask))
     abort();