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>

src/arch/x86/smbios.c

@@ -46,6 +46,13 @@ int smbios_add_string(char *start, const char *str)
 	int i = 1;
 	char *p = start;
 
+	/*
+	 * Return 0 as required for empty strings.
+	 * See Section 6.1.3 "Text Strings" of the SMBIOS specification.
+	 */
+	if (*str == '\0')
+		return 0;
+
 	for(;;) {
 		if (!*p) {
 			strcpy(p, str);