nfit: in acpi_nfit_init, break on a 0-length table

If acpi_nfit_init is called (such as from nfit_test), with an nfit table
that has more memory allocated than it needs (and a similarly large
'size' field, add_tables would happily keep adding null SPA Range tables
filling up all available memory.

Make it friendlier by breaking out if a 0-length header is found in any
of the tables.

Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: <linux-acpi@vger.kernel.org>
Cc: <linux-nvdimm@lists.01.org>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/acpi/nfit.c

@@ -335,6 +335,12 @@ static void *add_table(struct acpi_nfit_desc *acpi_desc, void *table,
 		return NULL;
 
 	hdr = table;
+	if (!hdr->length) {
+		dev_warn(dev, "found a zero length table '%d' parsing nfit\n",
+			hdr->type);
+		return NULL;
+	}
+
 	switch (hdr->type) {
 	case ACPI_NFIT_TYPE_SYSTEM_ADDRESS:
 		if (!add_spa(acpi_desc, table))