BTS_Tests: altstep as_meas_res: Check for meas timer timeout only if previously set

Should fix following spotted message in TTCN3 logs:
BTS_Tests.ttcn:1297 Timeout operation on timer g_Tmeas_exp failed: The timer is not started.

Change-Id: Ia28c40e8cb338ecc23f72474832307bba2b09503
diff --git a/bts/BTS_Tests.ttcn b/bts/BTS_Tests.ttcn
index f66a43f..aee081e 100644
--- a/bts/BTS_Tests.ttcn
+++ b/bts/BTS_Tests.ttcn
@@ -1294,7 +1294,7 @@
 		setverdict(fail, "Received unexpected MEAS RES ", rsl);
 		self.stop;
 		}
-	[] g_Tmeas_exp.timeout {
+	[g_Tmeas_exp.running] g_Tmeas_exp.timeout {
 		setverdict(fail, "Didn't receive expected measurement result")
 		self.stop;
 		}