IuUP_Emulation: Fix frame number wrap-around

As seen when running a test:
IuUP_Emulation.ttcn:150 Dynamic test case error: While RAW-encoding type '@IuUP_Types.IuUP_PDU': There are insufficient bits to encode '@IuUP_Types.IuUP_PDU_Type_0.frame_nr'

Change-Id: Ic9e69115c49545a7b9797d8f85568d5e91dfb329
diff --git a/library/IuUP_Emulation.ttcn b/library/IuUP_Emulation.ttcn
index 56384ee..b35602c 100644
--- a/library/IuUP_Emulation.ttcn
+++ b/library/IuUP_Emulation.ttcn
@@ -143,7 +143,7 @@
 			} else {
 				pdu := valueof(ts_IuUP_Type1(st.tx_next_frame_nr, 0, payload));
 			}
-			st.tx_next_frame_nr := st.tx_next_frame_nr + 1;
+			st.tx_next_frame_nr := (st.tx_next_frame_nr + 1) mod 16;
 		}
 	}
 	if (isvalue(pdu)) {