ussd: Fix test for RELEASE COMPLETE
A correcsponding change in libosmocore sets text[0] to '\0'.
The test for 0xFF could never have been true.
diff --git a/openbsc/src/libmsc/ussd.c b/openbsc/src/libmsc/ussd.c
index 76ee101..b413c83 100644
--- a/openbsc/src/libmsc/ussd.c
+++ b/openbsc/src/libmsc/ussd.c
@@ -51,7 +51,7 @@
memset(&req, 0, sizeof(req));
gh = msgb_l3(msg);
rc = gsm0480_decode_ussd_request(gh, msgb_l3len(msg), &req);
- if (req.text[0] == 0xFF) /* Release-Complete */
+ if (req.text[0] == '\0') /* Release-Complete */
return 0;
if (!strcmp(USSD_TEXT_OWN_NUMBER, (const char *)req.text)) {