check for osmo_fsm_register() error return values

Change-Id: I2028c82ac1b0421101c3f5d04fd999b65abdbf08
diff --git a/src/sgsn/gprs_mm_state_gb_fsm.c b/src/sgsn/gprs_mm_state_gb_fsm.c
index f6d680c..811f0c2 100644
--- a/src/sgsn/gprs_mm_state_gb_fsm.c
+++ b/src/sgsn/gprs_mm_state_gb_fsm.c
@@ -125,5 +125,5 @@
 
 static __attribute__((constructor)) void mm_state_gb_fsm_init(void)
 {
-	osmo_fsm_register(&mm_state_gb_fsm);
+	OSMO_ASSERT(osmo_fsm_register(&mm_state_gb_fsm) == 0);
 }