A potential NPE bug

Hi all, our bug scanner has reported a NPE bug.

A possible call chain may be [getNode()](https://github.com/crossoverJie/JCSprout/tree/master/src/main/java/com/crossoverjie/actual/LRUMap.java#L107) => [LRUMap.java#L65](https://github.com/crossoverJie/JCSprout/tree/master/src/main/java/com/crossoverjie/actual/LRUMap.java#L65) => [LRUMap.java#L68](https://github.com/crossoverJie/JCSprout/tree/master/src/main/java/com/crossoverjie/actual/LRUMap.java#L68). Since the method `getNode()` may return `null` at [LRUMap.java#L118](https://github.com/crossoverJie/JCSprout/tree/master/src/main/java/com/crossoverjie/actual/LRUMap.java#L118), a NPE bug may take place at [LRUMap.java#L76](https://github.com/crossoverJie/JCSprout/tree/master/src/main/java/com/crossoverjie/actual/LRUMap.java#L76) when the variable `node` is `null`.

Thanks.

Start by reading src/main/java/com/crossoverjie/actual/LRUMap.java around lines 65-76 and 107-118, following the reported getNode() call chain. Determine whether a null node can reach line 76, then add coverage for that path and verify the issue no longer produces an NPE.

Written by the indexing model from the issue text.