arch/x86/smbios: Check str for NULL in smbios_add_string()

Signed-off-by: Maximilian Brune <maximilian.brune@9elements.com>
Change-Id: Ic228b869aea362c1f07e0808c2735ff3b285a6bd
Reviewed-on: https://review.coreboot.org/c/coreboot/+/73980
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>

src/arch/x86/smbios.c

@@ -45,7 +45,7 @@ int smbios_add_string(u8 *start, const char *str)
 	 * Return 0 as required for empty strings.
 	 * See Section 6.1.3 "Text Strings" of the SMBIOS specification.
 	 */
-	if (*str == '\0')
+	if (str == NULL || *str == '\0')
 		return 0;
 
 	for (;;) {