GUACAMOLE-362: Avoid NullPointerException when closing input stream.

extensions/guacamole-auth-cas/src/main/java/org/apache/guacamole/auth/cas/conf/PrivateKeyGuacamoleProperty.java

@@ -81,6 +81,7 @@ public abstract class PrivateKeyGuacamoleProperty implements GuacamoleProperty<P
                 throw new GuacamoleServerException("Key is not in expected PKCS8 encoding.", e);
             }
             finally {
+                if (keyStreamIn != null)
                     keyStreamIn.close();
             }
         }