gprs_ns2: fix crash when changing the MTU

When the MTU changes for any fr device, all
NSE will recalculate their MTU. If any NSE is alive,
libosmocore will crash.

Related: OS#5192
Change-Id: I31ba5cefea7bbb0b74060d6664b42c58815ee2a1
diff --git a/src/gb/gprs_ns2.c b/src/gb/gprs_ns2.c
index 720e836..a8e646b 100644
--- a/src/gb/gprs_ns2.c
+++ b/src/gb/gprs_ns2.c
@@ -1552,7 +1552,7 @@
 
 	nse->mtu = mtu;
 	if (nse->alive)
-		ns2_prim_status_ind(nsvc->nse, NULL, 0, GPRS_NS2_AFF_CAUSE_MTU_CHANGE);
+		ns2_prim_status_ind(nse, NULL, 0, GPRS_NS2_AFF_CAUSE_MTU_CHANGE);
 }
 
 /*! calculate the transfer capabilities for a nse