ACPI / APEI: Fix crash in apei_hest_parse() for acpi=off

After commit 92ef2a25 (ACPI: Change the ordering of PCI root bridge
driver registrarion), acpi_hest_init() is never called for acpi=off
(acpi_disabled), so hest_disable is not set, but hest_tab is NULL,
which causes apei_hest_parse() to crash when it is called from
aer_acpi_firmware_first().

Fix that by making apei_hest_parse() check if hest_tab is not NULL
in addition to checking hest_disable.  Also remove the now useless
acpi_disabled check from apei_hest_parse().
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/apei/hest.c

@@ -89,7 +89,7 @@ int apei_hest_parse(apei_hest_func_t func, void *data)
 	struct acpi_hest_header *hest_hdr;
 	int i, rc, len;
 
-	if (hest_disable)
+	if (hest_disable || !hest_tab)
 		return -EINVAL;
 
 	hest_hdr = (struct acpi_hest_header *)(hest_tab + 1);
@@ -216,9 +216,6 @@ void __init acpi_hest_init(void)
 		return;
 	}
 
-	if (acpi_disabled)
-		goto err;
-
 	status = acpi_get_table(ACPI_SIG_HEST, 0,
 				(struct acpi_table_header **)&hest_tab);
 	if (status == AE_NOT_FOUND)