Check cpu_softc is not NULL before dereferencing

In the acpi_cpu_postattach SYSINIT function cpu_softc may be NULL, e.g.
on arm64 when booting from FDT. Check it is not NULL at the start of
the function so we don't try to dereference a NULL pointer.

Sponsored by:	The FreeBSD Foundation

sys/dev/acpica/acpi_cpu.c

@@ -444,6 +444,9 @@ acpi_cpu_postattach(void *unused __unused)
     struct acpi_cpu_softc *sc;
     int attached = 0, i;
 
+    if (cpu_softc == NULL)
+	return;
+
     mtx_lock(&Giant);
     CPU_FOREACH(i) {
 	if ((sc = cpu_softc[i]) != NULL)