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>