added some null checks for device instance

Change-Id: I84ab4365513a61bac4a5e852c8589fae3c828710

@@ -164,7 +164,9 @@ public class NetconfDevice {
                     + deviceInfo(), e);
         } finally {
             log.debug("Closing the session after successful execution");
-            ssh.close();
+            if (ssh != null) {
+                ssh.close();
+            }
         }
     }
 

@@ -339,7 +339,7 @@ public class NetconfDeviceProvider extends AbstractProvider
                         + device.deviceInfo(), e);
             } catch (Exception e) {
                 log.error("Error while initializing session for the device: "
-                        + device.deviceInfo(), e);
+                        + (device != null ? device.deviceInfo() : null), e);
             }
         }
 

@@ -378,7 +378,7 @@ public class NetconfDeviceProviderTest {
                 throw e;
             } catch (Exception e) {
                 log.error("Error while initializing session for the device: "
-                        + device.deviceInfo(), e);
+                        + (device != null ? device.deviceInfo() : null), e);
             }
         }