x86/smbios: Return index 0 for empty strings

Section 6.1.3 (Text Strings) of the SMBIOS specification states:
  If a string field references no string, a null (0) is placed in that
  string field.

Change smbios_add_string() to do that.

Change-Id: I9c28cb89dcfe2c8ef2366c23ee6203e15b7c2513
Signed-off-by: Ben Gardner <gardner.ben@gmail.com>
Reviewed-on: https://review.coreboot.org/12697
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
1 file changed