iohandler.c: Properly initialize sigaction struct

The code in qemu_init_child_watch() wasn't clearing the 'struct
sigaction' before passing it to sigaction(); this meant that we
would block a random set of signals while executing the SIGCHLD
handler. Initialize properly by using memset() on the struct,
as we do in similar cases elsewhere.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

iohandler.c

@@ -191,6 +191,7 @@ static void qemu_init_child_watch(void)
     struct sigaction act;
     sigchld_bh = qemu_bh_new(sigchld_bh_handler, NULL);
 
+    memset(&act, 0, sizeof(act));
     act.sa_handler = sigchld_handler;
     act.sa_flags = SA_NOCLDSTOP;
     sigaction(SIGCHLD, &act, NULL);