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: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Andrzej Zaborowski <andrew.zaborowski@intel.com>
