ussd: Add a test case, switch parsing to use a gsm48_hdr and len

The current USSD code is not doing any size checks, add a test
case to find out how easily we access the data out of bounds.
Begin to use the length in some places.
diff --git a/configure.in b/configure.in
index 4c827b0..a8e3f20 100644
--- a/configure.in
+++ b/configure.in
@@ -112,4 +112,5 @@
 	tests/timer/Makefile
 	tests/sms/Makefile
 	tests/msgfile/Makefile
+	tests/ussd/Makefile
 	Makefile)