Fix Jira 923 BLE 4.2 BLECharacteristic::setValue() hangs

Root cause
  1. The central powered off peripherial device and make it not give response.

Solution
  1. Exit the wait loop when connection lost.

Changed file
  BLECharacteristicImp.cpp - Change the wait logic

libraries/CurieBLE/src/internal/BLECharacteristicImp.cpp

@@ -750,10 +750,12 @@ bool BLECharacteristicImp::write(const unsigned char value[],
         retval = bt_gatt_write(conn, &params);
         if (0 == retval)
         {
+            bool connected = true;
             // Wait for write response
-            while (_gattc_writing)
+            while (_gattc_writing && connected)
             {
                 delay(2);
+                connected = _ble_device.connected();
             }
             write_process_result = _gattc_write_result;
         }