compatfd.c: Don't pass NULL pointer to SYS_signalfd

Don't pass a NULL pointer in to SYS_signalfd in qemu_signalfd_available():
this isn't valid and Valgrind complains about it.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NAndrzej Zaborowski <andrew.zaborowski@intel.com>

compatfd.c

@@ -119,9 +119,17 @@ int qemu_signalfd(const sigset_t *mask)
 bool qemu_signalfd_available(void)
 {
 #ifdef CONFIG_SIGNALFD
+    sigset_t mask;
+    int fd;
+    bool ok;
+    sigemptyset(&mask);
     errno = 0;
-    syscall(SYS_signalfd, -1, NULL, _NSIG / 8);
-    return errno != ENOSYS;
+    fd = syscall(SYS_signalfd, -1, &mask, _NSIG / 8);
+    ok = (errno != ENOSYS);
+    if (fd >= 0) {
+        close(fd);
+    }
+    return ok;
 #else
     return false;
 #endif