GSUP: add end marker to enum osmo_gsup_iei

Simplify gsup_test.c by defining an end marker in gsup.h. No need to
manually update the last element every time anymore.

The C standard guarantees, that the end marker will have the last value
plus one: "Each subsequent enumerator with no = defines its enumeration
constant as the value of the constant expression obtained by adding 1 to
the value of the previous enumeration constant." (From C99: 6.7.2.2
Enumeration specifiers)

Change-Id: I2aab7245e209f0ebd2f33a83d4d181dd3339cb17

include/osmocom/gsm/gsup.h

@@ -98,6 +98,8 @@ enum osmo_gsup_iei {
 	OSMO_GSUP_SM_RP_CAUSE_IE		= 0x44,
 	OSMO_GSUP_SM_RP_MMS_IE			= 0x45,
 	OSMO_GSUP_SM_ALERT_RSN_IE		= 0x46,
+
+	_OSMO_GSUP_IEI_END_MARKER
 };
 
 /*! GSUP message type */

tests/gsup/gsup_test.c

@@ -401,11 +401,7 @@ static void test_gsup_messages_dec_enc(void)
 					osmo_hexdump(t->data + j, ie_end - j));
 
 				OSMO_ASSERT(j <= ie_end - 2);
-				/**
-				 * FIXME: share the maximal IE value somehow
-				 * in order to avoid manual updating of this
-				 */
-				OSMO_ASSERT(t->data[j+0] <= OSMO_GSUP_SM_ALERT_RSN_IE);
+				OSMO_ASSERT(t->data[j+0] < _OSMO_GSUP_IEI_END_MARKER);
 				OSMO_ASSERT(t->data[j+1] <= ie_end - j - 2);
 
 				ie_end = j;