Fix a compare against undefined variable in acpi.c

Initialize the pointer fadt to NULL to prevent a later comparison
(if (fadt == NULL)) when the pointer had the *possibility* of never
having been initialized.

Change-Id: Ib2a544c190b609ab8c23147dc69dca5f4ac7f38c
Signed-off-by: Martin Roth <martin@se-eng.com>
Reviewed-on: http://review.coreboot.org/2037
Reviewed-by: Peter Stuge <peter@stuge.se>
Tested-by: build bot (Jenkins)

src/arch/x86/boot/acpi.c

@@ -687,7 +687,7 @@ void *acpi_find_wakeup_vector(void)
 	char *p, *end;
 	acpi_rsdt_t *rsdt;
 	acpi_facs_t *facs;
-	acpi_fadt_t *fadt;
+	acpi_fadt_t *fadt = NULL;
 	void *wake_vec;
 	int i;