ACPI/nfit: avoid accessing uninitialized memory in acpi_nfit_ctl()

The ACPI_ALLOCATE() does not zero the "buf", so when the condition
"integer->type != ACPI_TYPE_INTEGER" in int_to_buf() is met, the result
is unpredictable in acpi_nfit_ctl().

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/r/20201118073517.1884-1-thunder.leizhen@huawei.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>