check for osmo_fsm_register() error return values

Change-Id: I2028c82ac1b0421101c3f5d04fd999b65abdbf08
diff --git a/src/sgsn/gprs_gmm_attach.c b/src/sgsn/gprs_gmm_attach.c
index 130f8d1..c903a01 100644
--- a/src/sgsn/gprs_gmm_attach.c
+++ b/src/sgsn/gprs_gmm_attach.c
@@ -449,7 +449,7 @@
 
 static __attribute__((constructor)) void gprs_gmm_fsm_init(void)
 {
-	osmo_fsm_register(&gmm_attach_req_fsm);
+	OSMO_ASSERT(osmo_fsm_register(&gmm_attach_req_fsm) == 0);
 }
 
 void gmm_att_req_free(struct sgsn_mm_ctx *mm) {