SGSN_Tests: fix regression in f_send_l3_gmm_llc() which breaks TC_attach_rau_a_b

The gb_index was forgotten to given to the new function f_send_l3().
All testcases which only used the default BSSGB connection #0 continued
to work, but the TC_attach_rau_a_b is the only testcase which uses #0
and #1 at the same time.

Fixes: a05b80792266 ("sgsn: Add TC_llc_null to test if SGSN survives a LLC NULL packet")
Change-Id: Ie3dd8c613d3b3440447a282dc4545078cb927274
diff --git a/sgsn/SGSN_Tests.ttcn b/sgsn/SGSN_Tests.ttcn
index 6665dc0..d081360 100644
--- a/sgsn/SGSN_Tests.ttcn
+++ b/sgsn/SGSN_Tests.ttcn
@@ -385,7 +385,7 @@
 	var octetstring l3_enc := enc_PDU_L3_MS_SGSN(valueof(l3_mo));
 	var BIT4 sapi := f_llc_sapi_by_l3_mo(valueof(l3_mo));
 	var integer n_u := f_llc_get_n_u_tx(llc[bit2int(sapi)]);
-	f_send_llc(ts_LLC_UI(l3_enc, sapi, '0'B, n_u));
+	f_send_llc(ts_LLC_UI(l3_enc, sapi, '0'B, n_u), gb_index);
 }
 
 altstep as_mm_identity(integer gb_idx := 0) runs on BSSGP_ConnHdlr {