ACPI: Set handle to NULL.

There is a (probably non-viable) case where it might not get initialized
by the function and then potentially used, so set it to NULL to silence
Clang.

src/add-ons/kernel/bus_managers/acpi/Device.cpp

@@ -87,7 +87,7 @@ acpi_walk_resources(acpi_device device, char *method,
 static status_t
 acpi_device_init_driver(device_node *node, void **cookie)
 {
-	ACPI_HANDLE handle;
+	ACPI_HANDLE handle = NULL;
 	const char *path = NULL;
 	uint32 type;