tests/v110: assert(user_data_chunk_bits) in test_ra1()

Coverity warns that osmo_v110_sync_ra1_get_user_data_chunk_bitlen() may
return a negative value, which is used as loop boundary.  Even though
this is unlikely, let's add an assert().

Change-Id: I0fc0e0bac74bd96351030432ef1b140b727acb0d
Fixes: CID#310968

tests/v110/test_ra1.c

@@ -10,7 +10,7 @@
 static void test_ra1(enum osmo_v100_sync_ra1_rate rate)
 {
 	int user_rate = osmo_v110_sync_ra1_get_user_data_rate(rate);
-	int user_data_chunk_bits = osmo_v110_sync_ra1_get_user_data_chunk_bitlen(rate);
+	int user_data_chunk_bits;
 	struct osmo_v110_decoded_frame fr;
 	ubit_t user_bits[48];
 	ubit_t bits[80];
@@ -19,6 +19,9 @@ static void test_ra1(enum osmo_v100_sync_ra1_rate rate)
 
 	printf("\n======= User data rate %u\n", user_rate);
 
+	user_data_chunk_bits = osmo_v110_sync_ra1_get_user_data_chunk_bitlen(rate);
+	OSMO_ASSERT(user_data_chunk_bits >= 0);
+
 	/* we abuse the fact that ubit_t is 8bit so we can actually
 	 * store integer values to clearly identify which bit ends up where */
 	memset(user_bits, 0xFE, sizeof(user_bits));