Fix error in tests-mbed_drivers-ticker when LED not available

Author: cypress-neil

TESTS/mbed_drivers/ticker/main.cpp

@@ -71,7 +71,9 @@ void ticker_callback_1(void)
 void ticker_callback_2(void)
 {
     ++callback_trigger_count;
-    switch_led2_state();
+    if (LED2 != NC) {
+        switch_led2_state();
+    }
 }
 
 
@@ -110,7 +112,9 @@ void test_case_1x_ticker()
     Ticker ticker;
 
     led1 = 1;
-    led2 = 1;
+    if (LED2 != NC) {
+        led2 = 1;
+    }
     callback_trigger_count = 0;
 
     greentea_send_kv("timing_drift_check_start", 0);
@@ -151,7 +155,9 @@ void test_case_2x_ticker()
     Ticker ticker1, ticker2;
 
     led1 = 0;
-    led2 = 1;
+    if (LED2 != NC) {
+        led2 = 1;
+    }
     callback_trigger_count = 0;
 
     ticker1.attach_us(ticker_callback_1, 2 * ONE_MILLI_SEC);