smbios: if a string value is unknown, use string number 0

smbios: if a string value is unknown, use string number 0

The SMBIOS specification describes: "If a string field references no string,
a null (0) is placed in that string field."

Accordingly we should avoid writing a string "Unknown" to the SMBIOS table.

dmidecode displays 'Not Specified' if the string number is 0.

Commit 00a871d34e2f ("smbios: empty strings in smbios_add_string()")
correctly identified that strings may not have length 0 as two
consecutive NULs indentify the end of the string list. But the suggested
solution did not match the intent of the SMBIOS specification.

Fixes: 00a871d34e2f ("smbios: empty strings in smbios_add_string()")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>