cmd: smbios: replace missing string by 'Not Specified'

A missing string value is indicated by a string index of 0. In this case
print 'Not Specified' like the Linux dmidecode command does.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/cmd/smbios.c b/cmd/smbios.c
index 62935ec..95bdff6 100644
--- a/cmd/smbios.c
+++ b/cmd/smbios.c
@@ -25,6 +25,10 @@
 {
 	const char *str = (char *)table +
 			  ((struct smbios_header *)table)->length;
+	static const char fallback[] = "Not Specified";
+
+	if (!index)
+		return fallback;
 
 	if (!*str)
 		++str;
@@ -41,7 +45,7 @@
 	if (table->type == SMBIOS_END_OF_TABLE)
 		return NULL;
 
-	str = smbios_get_string(table, 0);
+	str = smbios_get_string(table, -1);
 	return (struct smbios_header *)(++str);
 }