SGSN_Tests: Only call f_bssgp_client_llgmm_assign for Gb RAN

Calling this function with Iu RAN (ran_index > 2) does not work because
BSSGP_PROC[ran_index] doesn't exist.

Change-Id: Ieef654e3886759a40763ece3903de9127ba8d9c6
Related: OS#3727
diff --git a/sgsn/SGSN_Tests.ttcn b/sgsn/SGSN_Tests.ttcn
index f070429..800c978 100644
--- a/sgsn/SGSN_Tests.ttcn
+++ b/sgsn/SGSN_Tests.ttcn
@@ -677,7 +677,9 @@
 	/* update TLLI */
 	g_pars.tlli_old := g_pars.tlli;
 	g_pars.tlli := g_pars.p_tmsi or4b 'c0000000'O;
-	f_bssgp_client_llgmm_assign(g_pars.tlli_old, g_pars.tlli, BSSGP_PROC[ran_index]);
+	if (is_gb(ran_index)) {
+		f_bssgp_client_llgmm_assign(g_pars.tlli_old, g_pars.tlli, BSSGP_PROC[ran_index]);
+	}
 }
 
 function f_process_attach_accept(PDU_GMM_AttachAccept aa) runs on BSSGP_ConnHdlr {