Fixes #664 do not recycle (requite) PoolEntrys that have been closed.

src/main/java/com/zaxxer/hikari/pool/PoolEntry.java

@@ -82,8 +82,10 @@ final class PoolEntry implements IConcurrentBagEntry
     */
    void recycle(final long lastAccessed)
    {
-      this.lastAccessed = lastAccessed;
+      if (connection != null) {
-      hikariPool.releaseConnection(this);
+         this.lastAccessed = lastAccessed;
+         hikariPool.releaseConnection(this);
+      }
    }
 
    /**