Erasure Coding: The length of targets should be checked when DN gets a reconstruction task

The length of targets should be checked when DN gets a EC reconstruction task.For some reason (HDFS-14768, HDFS-16739) , the length of targets will be larger than additionalReplRequired which causes some elements in targets get the default value 0. It may trigger the bug which leads to the data corrupttion just like HDFS-14768.