libvlr: fix wrong cause value passed to auth_fsm_term()

This was found thanks to clang (-Wenum-conversion):

warning: implicit conversion from enumeration type
         'enum gsm48_gmm_cause' to different enumeration type
         'enum gsm48_reject_value' [-Wenum-conversion]

Change-Id: I0b820bb2a8e561682a8158fc51bd9565f5912d56
diff --git a/src/libvlr/vlr_auth_fsm.c b/src/libvlr/vlr_auth_fsm.c
index 14345b2..5207464 100644
--- a/src/libvlr/vlr_auth_fsm.c
+++ b/src/libvlr/vlr_auth_fsm.c
@@ -391,7 +391,8 @@
 	case VLR_AUTH_E_HLR_SAI_NACK:
 		/* HLR did not return Auth Info, hence cannot authenticate. (The caller may still decide to permit
 		 * attaching without authentication) */
-		auth_fsm_term(fi, AUTH_FSM_NO_AUTH_INFO, gsup->cause);
+		vlr_gmm_cause_to_mm_cause(gsup->cause, &gsm48_rej);
+		auth_fsm_term(fi, AUTH_FSM_NO_AUTH_INFO, gsm48_rej);
 		break;
 	case VLR_AUTH_E_HLR_SAI_ABORT:
 		vlr_gmm_cause_to_mm_cause(gsup->cause, &gsm48_rej);