[Hot Fix #42] Do not stop SparkUI if bind() is not called

This is a bug fix for #42 (79d07d66040f206708e14de393ab0b80020ed96a).

In Master, we do not bind() each SparkUI because we do not want to start a server for each finished application. However, when we remove the associated application, we call stop() on the SparkUI, which throws an assertion failure.

This fix ensures we don't call stop() on a SparkUI that was never bind()'ed.