[SPARK-6762]Fix potential resource leaks in CheckPoint CheckpointWriter and CheckpointReader

The close action should be placed within finally block to avoid the potential resource leaks

Author: lisurprise <zhichao.li@intel.com>

Closes #5407 from zhichao-li/master and squashes the following commits:

065999f [lisurprise] add guard for null
ef862d6 [lisurprise] remove fs.close
a754adc [lisurprise] refactor with tryWithSafeFinally
824adb3 [lisurprise] close before validation
c877da7 [lisurprise] Fix potential resource leaks